Feeding after lights out: the bottom-dwellers that starve, and the food that fouls the tank overnight

Quick answer

The evening tank. What happens in the hours after the light goes off decides both whether the bottom-dwellers eat and whether the water is still good in the morning.

Leaving food in a tank so that fish can graze at will is not a feeding strategy. Food that is not eaten decays, and decaying food consumes oxygen and releases ammonia at the exact time of day when a tank has least oxygen to spare.

The opposite problem is real, though, and it is the one that quietly kills fish. Bottom-dwelling and night-active species are routinely outcompeted by faster day-active fish, and their owners conclude they are not hungry because they never see them eat. The answer is a deliberate, timed, measured feed after the lights go off, not food left down all night.

• Feed the day fish first, turn the lights off, wait, then place sinking food where the bottom-dwellers actually live.
• Measure the portion. Anything uneaten by morning was too much, and it should come out.
• A "clean-up crew" fish is not fed by the tank. Plecos and loaches starve slowly in clean aquariums.
• Overnight is the daily oxygen minimum in any planted or warm tank, so a heavy night feed is worse than the same feed given in the morning.
• In hot weather, and in an unheated pond as the water cools, the whole feeding plan changes.

:::danger
Nothing in an aquarium feeds itself.

Plecos, otocinclus, loaches and shrimp are sold as algae eaters and clean-up crews, and owners then feed them nothing. Algae in a well kept tank is not enough to sustain a growing pleco, and a bare glass tank produces almost none at all.

Starvation in these fish is slow and quiet. The belly goes flat, then concave, the area behind the head hollows, and the fish is often dead months after the mistake was made. By then nobody connects the two.

Every fish in the tank has a diet. If you cannot name what a fish is eating, it is not eating.
:::

Decide in 60 seconds

Why overnight is the wrong time for leftovers

A planted tank produces oxygen during the day and consumes it at night. Once the lights go off, photosynthesis stops but respiration does not, so plants, fish and the whole bacterial population draw oxygen down for hours without anything replacing it.

The lowest dissolved oxygen reading in a twenty-four hour cycle is therefore shortly before the lights come on. That is why a tank that looks fine all day can have fish at the surface at six in the morning.

Uneaten food lands on top of that. Bacteria breaking it down consume oxygen while doing so, and release ammonia as they go. Both effects arrive during the hours when the tank has the least margin.

Temperature makes it worse in both directions. Warm water holds less dissolved oxygen and speeds up decay, so the same forgotten wafer does more damage in summer. In a heatwave, an evening feed that was harmless in spring can be the difference between a normal night and a tank of gasping fish.

None of this argues for underfeeding. It argues for feeding a measured amount that gets eaten, at a time when you are there to see whether it was.

Why the bottom-dwellers miss out

A Corydoras working the substrate. Barbels find food by taste and touch, which is thorough and slow. Visual feeders in the upper water get to the same food first.

Feeding at the top of the tank is a competition, and it is won on speed. Tetras, barbs, danios and livebearers see food hit the water and take it within seconds.

Sinking wafers are supposed to solve this, and often do not, because a wafer takes time to fall and mid-water fish intercept it on the way down. What reaches the substrate is what nobody wanted.

The bottom-dwellers themselves are built for a different feeding style. Catfish barbels are chemosensory, so the fish tastes its way across the substrate in a methodical search. That works well in a river bed at night and works badly against a shoal of hungry tetras at four in the afternoon.

Activity rhythm compounds it. Kuhli loaches, many plecos, synodontis and banjo catfish are genuinely nocturnal or crepuscular and are simply not out at feeding time. Corydoras are more flexible and often forage in daylight in a settled tank, but they are still bottom feeders taking second choice.

The substrate finishes the job. Coarse gravel swallows small food particles into gaps that no fish can reach, where it rots. Sand keeps food on the surface where bottom-dwellers can find it, which is one of several reasons sand suits Corydoras better.

The after-dark feed, done properly

Feed the community first, lightly.

Give the day-active fish their normal meal in the evening so they are not hunting for more.

Turn the light off and wait.

Give it a while, long enough for the upper-tank fish to settle and for the nocturnal species to start moving. There is no exact figure. Watch your own tank and learn how long it takes.

Place the food where the fish lives.

Drop sinking wafers or pellets in the corner, cave mouth or wood pile the fish occupies, not in the middle where the current takes them. Feeding tongs or a length of rigid airline tubing gets the food to the bottom without it drifting.

Use a feeding dish on gravel.

A shallow dish keeps food on top of the substrate rather than in it, keeps it in one findable place, and makes leftovers easy to remove. On sand it matters less.

Watch with a dim light, not the tank light.

A torch pointed at the ceiling to bounce light into the room is usually enough. Switching the main light back on defeats the purpose.

Measure and check.

Note what you put in. Look in the morning. If food remains, reduce next time.

Match the food to the fish.

Bristlenose and other rasping plecos need vegetable matter and, in wood-eating species, real driftwood to graze. Blanched courgette, cucumber, sweet potato and similar vegetables are taken readily, weighted down with a vegetable clip or a stainless fork, and removed after a few hours rather than left overnight.

Loaches and many catfish are omnivores or carnivores and want protein: sinking carnivore pellets, frozen bloodworm or similar, rather than algae wafers.

Otocinclus are specialised grazers with small stomachs and need continuous soft algae and biofilm, plus supplementary vegetable, in a tank mature enough to provide it.

What healthy body condition looks like

You will not usually watch these fish eat, so body condition is the measurement that matters.

Look at the fish from the side, against the glass, with the light behind you. The belly should be gently rounded from the gill covers to the vent, in a continuous line.

A flat belly means it is eating less than it needs. A concave belly, or a hollow just behind the head where the muscle has gone, means it has been underfed for a long time and is drawing on its own tissue.

In plecos, look at the width of the body behind the head from above. In loaches, look for a thin caudal peduncle, the section of body just before the tail.

Check barbels on any barbelled fish. They should be full length, straight and mobile. Short, stubby or absent barbels mean physical damage from sharp substrate or chemical damage from detritus and poor water, and a fish with damaged barbels finds less food than one with intact ones.

Growth is the other measure in young fish. A juvenile bristlenose that has not visibly grown in months is not being fed enough, whatever the tank looks like.

The seasonal picture

Summer, indoors.

Warm water holds less oxygen and speeds decomposition. Feed smaller amounts, feed earlier, increase surface agitation, and be stricter about removing leftovers. If fish are at the surface in the early morning, cut the evening feed before you change anything else.

Summer, outdoors.

Ponds warm dramatically and algal blooms consume oxygen overnight, exactly as tank plants do. A pond with a heavy bloom and a heavy evening feed is the classic overnight fish loss.

Autumn and winter, outdoors.

Fish are ectotherms, so digestion slows with the water temperature. Food that would be digested in a day in summer sits in the gut in cold water. Pond keepers commonly move to a wheatgerm-based food formulated for cool water as the season turns, and stop feeding altogether once the water is consistently cold. Judge it by water temperature, not by the calendar or by whether the fish come to the surface.

Winter, indoors.

A heated tank is stable, but a room that gets cold at night can pull an under-powered heater below its set point. Check the temperature at night as well as during the day if fish suddenly go off food in winter.

Any season, during a power cut.

Do not feed. Circulation and aeration have stopped and the tank is already losing oxygen.

Getting this right, and keeping it right

What never to do

Never leave food in the tank overnight as a policy. Uneaten food is an ammonia source, not a buffet.

Never assume an algae eater is fed by the tank. This is the commonest way a pleco dies in a well maintained aquarium.

Never feed heavily on a hot evening. The overnight oxygen minimum is deeper in warm water and a large feed makes it deeper still.

Never bury food in gravel to get it past the top fish. It rots where nothing can reach it and produces exactly the water problem you were avoiding.

Never keep Corydoras or other barbelled catfish on sharp gravel and then wonder why they feed poorly.

Never judge a nocturnal fish's appetite by whether you see it eat. Judge it by body condition and by whether food is gone in the morning.

Never continue feeding a pond into cold weather because the fish still come up. They will take food they cannot digest.

When to get help

Resting at night is normal. Fish do not have eyelids, so a fish that is still, low in the tank and holding position is asleep rather than unwell. What matters is whether it resumes normal behaviour when the light returns.

Contact an aquatic vet or an experienced specialist the same day if fish are gasping at the surface in the morning, if several fish stop feeding at once, or if a fish that was eating stops entirely.

Book advice sooner rather than later for a bottom-dweller with a concave belly, for barbels that are eroding despite a substrate change, or for a fish that is losing condition while apparently eating.

Bring the tank volume and age, the full stocking list, exactly what is fed and when, the substrate type, whether food remains in the morning, water test results including ammonia, nitrite and nitrate, and the temperature range across a full day and night.

Photographs of body condition, taken from the side and from above in consistent lighting a few weeks apart, are more useful than any description. Weight loss in fish is slow enough that a comparison is what reveals it.