Your culture is struggling.
Here is what to do about it.
Practical, honest troubleshooting for copepod, phytoplankton, and rotifer cultures. Written from 18+ years of running live marine cultures in Wales.
Most culture problems have the same small number of causes.
In 18+ years of culturing live marine food, the patterns repeat. A crashed copepod culture almost always comes down to one of four things. A declining phytoplankton culture failing to stay green traces back to the same root causes time and again. Understanding which problem you actually have is the first step to fixing it quickly and getting back on track.
Use the sections below to navigate to your culture type, find the symptom that matches what you are seeing, and follow the practical steps. If you work through the guidance and you are still stuck, Darren answers personally by phone or email.
Darren, Director, Reefphyto Ltd
Get these right and most problems will not occur.
Temperature stability
Fluctuating temperature is a culture killer. Sudden drops or spikes stress animals immediately. Aim for 20-24°C and keep it consistent.
Light quality and duration
Phytoplankton needs steady, bright light. Copepods and rotifers need indirect light only. Wrong light on the wrong culture causes rapid decline.
Feeding rate
Overfeeding is as damaging as underfeeding. Excess food rots, crashes water quality, and wipes cultures quickly. Feed to the visual cue, not a fixed volume.
Clean equipment
Rinse culture vessels with fresh water only, never chemical detergent. Even trace soap residue is lethal to rotifers.
What are you troubleshooting?
Copepod culture troubleshooting
Tigriopus californicus is a robust, forgiving copepod when the basics are right. Most culture problems trace back to feeding errors, temperature shock, or contamination introduced during a water change. Find your symptom below.
A white or milky culture almost always means a bacterial bloom triggered by overfeeding or a temperature spike. The bacteria have outcompeted everything else and the oxygen level has likely dropped. This is the most common copepod crisis, and it is recoverable if you act quickly.
Do not wait to see if it recovers on its own. A bacterial bloom that is left to run will wipe the culture completely within 24 to 48 hours.
What to do: Pour the culture through a fine sieve to recover any surviving animals. Rinse gently with saltwater at the correct salinity. Transfer the rescued animals into a clean vessel with fresh saltwater. Do not feed for 48 hours. When you resume, start at a fraction of your previous dose and build up gradually.
The safest guide for feeding copepods is colour. The water should carry a very faint green tint from phytoplankton. Clear water means they need feeding. Strongly coloured or murky water means you are overfeeding.
Declining numbers without a visible crash usually means one of three things: insufficient nutrition, temperature below optimal for reproduction, or population ageing without successful recruitment.
Check nutrition first. Tigriopus californicus breeds most actively when well-fed on a reliable phytoplankton source. If you have been using the same batch of feed for more than three to four weeks, the nutritional quality may have degraded. A fresh phytoplankton feed often triggers rapid breeding within days.
Check temperature. Tigriopus will survive at 18°C but reproduce significantly faster at 22 to 24°C. If your culture vessel sits in a cool room or near a window in winter, low temperature may be limiting recruitment.
Check the age of your culture. Cultures running for many months without a refresh can become stagnant. Adding a small volume of fresh stock every two to three months prevents this.
If you can see egg sacs attached to females, reproduction is happening. Wait another five to seven days before concluding recruitment has failed. Copepod development from egg to juvenile takes one to two weeks at optimal temperature.
This is one of the most common questions at the three to four week mark and it is usually not a problem at all. Copepod cultures do not grow on a straight upward line. Tigriopus californicus has a generation time of roughly two to three weeks at 22 to 24°C, which means the first real population increase often does not become visible until the second or third generation starts reproducing.
In the first few weeks you may feel like nothing much is happening. You can see adults, perhaps some egg sacs, but the vessel does not look significantly more populated than when you started. This is normal. The growth pattern is slow at first and then accelerates as more females reach breeding age simultaneously.
Expect the first noticeable population increase at around three to four weeks. By six to eight weeks a well-maintained culture should be visibly dense with animals at all life stages. If you are past eight weeks and still seeing very low numbers, revisit nutrition and temperature rather than assuming the culture has failed.
What helps at this stage: Consistent feeding, stable temperature, and patience. Avoid the temptation to harvest early. Let the population build to a level where removing animals does not set back the breeding momentum.
This is the question that means things are going right. The key is not to harvest too early or too aggressively, because taking too many animals before the population is fully established will stall breeding momentum and set you back weeks.
When to start: Wait until you can see dense numbers of adults, juveniles, and nauplii throughout the vessel. A culture that is ready for harvesting looks visibly busy at all levels of the water and on the base of the vessel. If you are mainly seeing adults with few juveniles, the population is not yet self-sustaining enough to handle regular removal.
How much to take: A safe rule is to harvest no more than 20 to 25% of the visible population at any one time. This leaves the breeding core intact and allows the culture to recover its numbers within a week or so.
Use a fine sieve to collect animals from the culture. Rinse gently into your tank or refugium with a small amount of tank water. Do not pour culture water directly into your display tank as it carries dissolved organics and waste products that belong in the culture vessel, not your reef.
If you are harvesting regularly and want to maintain a consistent supply, consider splitting your culture across two vessels once the population is established. This gives you a rotation where one vessel is always building numbers while the other is being harvested.
Yes, and it is a good idea once the population is large enough to handle it. Splitting too early is the risk. If you divide a culture that is still building its numbers, you end up with two weak cultures instead of one strong one, and both will take longer to reach productive density.
When to split: Wait until your original culture is visibly dense with animals at all life stages, adults, juveniles, and nauplii. You should be at the point where you could comfortably harvest from it. If the culture still feels like it is growing towards that level, it is too early to split.
How to split safely: Prepare a second vessel with fresh saltwater matched to the same salinity and temperature as your existing culture. Transfer roughly half the culture volume, animals and water together, into the new vessel. Top both vessels up with fresh saltwater. Feed both lightly for the first few days and allow them to re-establish before resuming your normal feeding routine.
Running two cultures gives you resilience. If one crashes you have a backup. It also allows you to harvest from one while the other recovers, giving you a more consistent supply of copepods for your tank.
Tigriopus californicus is one of the more forgiving species in this situation. A healthy, well-established copepod culture can tolerate three to five days without feeding or water changes and usually come through it without a major setback. The animals slow down their activity and metabolism, but they are hardy enough to survive a short gap in maintenance.
What to check when you return: Look at the water first. If it is still clear to faintly green and smells clean, the culture is almost certainly fine. Resume your normal feeding and water change routine and it will carry on as before.
If the water has gone cloudy, yellowish, or smells noticeably worse than usual, the culture has deteriorated while you were away. Do a partial water change of around 20 to 25% with fresh saltwater, hold off feeding for 24 hours, and then resume at a reduced rate. In most cases the culture will bounce back within a week.
This means a bacterial bloom has taken hold. Follow the crash recovery guidance in the first question above. Sieve out surviving animals, discard the water, and restart in a clean vessel. Even in this scenario you can usually rescue enough animals to rebuild without needing a completely new culture.
Before you go away: If you know you will be away for more than a couple of days, do a water change and a light feed just before you leave. This gives the culture the best possible starting point for the gap in maintenance. Avoid the temptation to overfeed before leaving as a way of compensating. Excess food in an unattended culture is more dangerous than no food at all.
For a healthy, actively reproducing culture, replacing around 20% of the water every seven to ten days is a reliable routine. This removes accumulated waste products, excess uneaten feed, and dissolved organics before they build up to levels that stress the animals.
Always replace with saltwater at the same salinity as the culture. A sudden salinity shift, even a small one, will stress copepods immediately and can trigger a crash in a culture that was otherwise healthy. Match temperature as closely as possible before adding the new water.
If the water is developing a yellowish tint, a stronger smell than usual, or you are seeing a surface film building up, your waste products are accumulating faster than your schedule accounts for. Increase to every five to seven days or reduce your feed rate, or both.
Do not change more than 30% of the water at one time. Large water changes remove a significant portion of the copepod nauplii and eggs that are suspended in the water column, which sets back recruitment and slows population recovery.
The water should remain clear to faintly green, smell clean and faintly marine, and show active animals at all levels of the vessel. If it meets all three of these between changes, your schedule is working well.
A foul smell from a copepod culture is a reliable sign of a serious water quality problem. A healthy culture should smell like clean, faintly marine water. A strong, sulphurous, or rotting smell means significant bacterial decomposition, almost certainly from accumulated uneaten food or dead animals.
At this point the culture may still be recoverable but you need to act immediately. Sieve out the animals, discard the water completely, rinse the vessel thoroughly with fresh water (no soap), and restart with clean saltwater. Do not attempt to partial-change your way out of this level of deterioration. A full reset is faster and safer.
Remove about 20% of the culture water and replace with fresh saltwater every seven to ten days. This regular dilution removes waste products before they accumulate to crisis levels.
Live animals in transit experience temperature fluctuation and oxygen consumption. Some reduction in activity on arrival is normal and usually temporary.
Before opening: Float the sealed bag in a vessel of saltwater at the correct salinity for 15 to 20 minutes to allow the temperature to equalise gradually. Sudden temperature change on opening is the most common cause of post-arrival mortality.
After temperature matching: Open slowly and introduce the culture to your vessel or refugium over ten minutes. Do not pour the entire volume in at once.
If animals appear dead on arrival: Contact Darren directly. We stand behind every culture we dispatch. Do not discard anything before contacting us.
Introduce copepods at lights-out or in dim conditions. Animals distribute naturally in low light and are less likely to be immediately predated.
Phytoplankton culture troubleshooting
A healthy phytoplankton culture is dense, richly green, and should smell fresh and slightly marine. Problems with phytoplankton are usually visible quickly, which makes them easier to catch early if you know what to look for.
A yellowing or paling phytoplankton culture is almost always a light problem. Phytoplankton is photosynthetic. Without adequate, consistent light, the cells stop producing chlorophyll and the culture bleaches. This is one of the most common issues home culturists encounter, particularly in winter when ambient light falls.
Check your light source. Phytoplankton cultures need bright, direct artificial light for 16 to 18 hours per day. A standard room light is rarely sufficient. If you are using a purpose-made culture lamp and the culture is still yellowing, the bulb may have degraded or the vessel may be placed too far from the light source.
Check your photoperiod. Consistency matters. Irregular light cycles stress the culture. Put it on a timer.
A pale culture still has living cells and can recover with improved light. A completely white culture has crashed and the cells are dead. The difference between the two is usually a matter of days.
If the culture has fully bleached, start with a clean vessel, clean water, and a fresh starter culture. Attempting to revive a crashed culture in the same vessel usually fails.
→ Fresh starter cultures available from Reefphyto if you need to restartBrown or orange colouration in a phytoplankton culture strongly suggests contamination, most likely from diatoms or dinoflagellates entering the culture. This is a known challenge in home culture systems because complete sterility is difficult to maintain.
Low levels of contamination can be tolerated and the green species may outcompete the intruders if conditions favour them. High nutrient media, good light, and good aeration all favour the green phytoplankton species in a blend.
If the brown or orange is spreading rapidly, the contaminant is likely winning. At this point a full restart is more reliable than trying to correct in place. Discard the current volume, sterilise the vessel with a dilute bleach solution followed by a thorough fresh water rinse, allow to dry completely, and restart with a fresh culture.
The most common route for contaminants into a phytoplankton culture is the air supply. Run the air through an inline micron filter. This single step eliminates the majority of airborne contamination.
A culture that maintains colour but will not increase in density is usually hitting a nutrient limitation or a CO2 limitation.
If you are using RO water with no added nutrients, the culture will stall once the initial nutrient supply is exhausted. A purpose-made phytoplankton culture medium or f/2 nutrient solution provides the minerals phytoplankton needs to divide actively.
Check your aeration. Gentle bubbling provides both mixing and CO2 exchange. A culture with no aeration will exhaust its dissolved CO2 and growth will halt even in good light with good nutrients.
Check your harvest rate. Leave at least 30 to 40% of the culture volume when harvesting to ensure robust regrowth.
This is worth understanding because it is actually a sign of quality rather than a problem. At Reefphyto we harvest phytoplankton at peak nutritional density - the point at which cell counts and fatty acid content are at their highest. At this stage the culture is genuinely dense and rich, but the colour can appear lighter than customers sometimes expect from photographs.
What arrives in your bottle is not a dilute or degraded culture. It is a concentrated, living product at the point of maximum nutritional value. The colour you see on arrival is the correct colour for a properly harvested, high-density phytoplankton culture.
Hold the bottle up to a light source. A healthy, dense culture will appear opaque - light should not pass through it clearly. Opacity indicates high cell density, which is exactly what you want. A watery, transparent bottle would be a concern. An opaque one is not.
Place the culture under your culture light on arrival and it will maintain and build on that density. If after 48 hours under good light you have any concerns about what you received, contact Darren directly with your order reference and he will advise.
Clumping or aggregation in a phytoplankton culture can be caused by several things. Some species in a multi-species blend naturally form loose colonies and this is normal. However, heavy settling or surface films can indicate a problem with aeration or an early sign of culture ageing.
If the culture is not being aerated, cells will settle. Gentle, continuous bubbling keeps cells suspended and ensures even light distribution throughout the culture volume.
If the culture is well-aerated but still aggregating heavily, this may indicate a culture that has been running for a long time without dilution or refreshing. Dilute with fresh culture medium to reduce density and the aggregation usually resolves.
A thin iridescent film on the surface is usually a biofilm of bacteria and is a sign the culture is ageing or slightly overfed. Gentle mixing and a partial water change usually clears this.
Rotifer culture troubleshooting
Rotifers are the most sensitive of the three cultures covered here. They respond quickly to water quality and feeding conditions, which means problems appear fast but so do recoveries if you act correctly. The key is catching issues early.
Rotifer crashes are often fast and total. Because Brachionus plicatilis reproduces by parthenogenesis and populations build quickly, they also crash quickly when conditions deteriorate. The most common causes are overfeeding, a sudden temperature change, and chemical contamination.
Identify the likely cause before restarting. Did you recently change your feed volume? Has the temperature changed suddenly? Was any equipment recently cleaned with soap or cleaning products?
Even trace amounts of detergent residue in a vessel will wipe a rotifer culture quickly. Always rinse equipment with fresh water only and allow to air-dry before use with a live culture.
Recovery: If you can see any surviving rotifers, sieve them out gently and transfer to a clean vessel. If the culture is completely wiped, restart from a new culture. Keep a small backup culture running in a separate vessel at all times if you rely on rotifers for breeding work.
→ Restart cultures available from Reefphyto — fresh, live Brachionus plicatilisSlow decline in a rotifer culture is almost always a feeding or water quality problem developing gradually. Rotifers need reliable, high-quality nutrition to maintain their reproduction rate above their natural mortality rate.
Check your feed quality. Rotifers fed exclusively on yeast-based feeds or low-quality phytoplankton will produce smaller, less nutritious animals and their reproduction rate will decline over time. A good live phytoplankton or a high-quality concentrated algae feed maintains both population density and animal quality.
Check your water change frequency. Ammonia and nitrite accumulate in a rotifer culture faster than in a copepod culture. A 50% water change every five to seven days is a sensible routine for an active culture.
Check for resting eggs. When conditions deteriorate, rotifers produce resting eggs (cysts) rather than active young. If you see a fine sediment developing at the bottom of the vessel and fewer active animals, the population is responding to stress. Improve conditions and the active population will recover from these cysts over several days.
→ Reefphyto Rotifer Feed Concentrate — Nannochloropsis oculata and Tetraselmis suecica blendRotifers that are present but noticeably smaller or sluggish are usually nutritionally stressed. Brachionus plicatilis is a continuous feeder and its body condition reflects the quality of its recent diet very directly.
This matters particularly if you are using rotifers for fish breeding. Small, slow rotifers fed to larvae deliver far less EPA, DHA, and other essential fatty acids than well-fed animals. The quality of the rotifer directly becomes the quality of the larval nutrition.
Switch to a high-quality phytoplankton feed immediately and maintain it consistently. Within four to five days, animal size and activity should visibly improve. If you need to enrich rotifers before a specific feeding session, a short-term enrichment period of four to six hours on a dedicated enrichment product immediately before harvesting will maximise their nutritional value.
Whatever is in the rotifer when it is eaten goes directly into your larvae or fish. A well-fed rotifer is not just bigger, it is a different nutritional proposition entirely.
Surface foam in a rotifer culture is common, particularly in cultures with aeration. A light, intermittent foam is usually caused by dissolved organic compounds from the feed and is not a problem.
A thick, persistent foam indicates elevated dissolved organics, almost always from overfeeding or inadequate water changes. Skim off the foam gently, perform a 40 to 50% water change with fresh saltwater, reduce your feed dose for the following few days, and monitor. The foam should reduce significantly within 24 to 48 hours.
Persistent foam in a culture that is not being overfed can also indicate that the culture has been running for a long time without a full clean of the vessel. A periodic full clean, while saving your animals in a temporary vessel, often resolves persistent foam in long-running cultures.
Rotifers are small enough that you cannot assess them easily with the naked eye. The water in the culture should be slightly turbid from dense numbers of animals.
The best way to check: Take a small sample, place it under a magnifying glass or low-power microscope, and look for movement. Healthy rotifers are visibly motile with their corona (feeding wheel) actively spinning.
If you do not have magnification, place a small volume in a shallow white dish and hold it up to a bright light. In a dense, healthy culture you may be able to see the collective movement of animals as a subtle turbulence in the water.
If after temperature matching and 24 hours in culture conditions you see no activity at all, contact Darren with your order details. Do not add a culture you suspect is dead to your breeding system.
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