Leaving fish while you travel: feeders, fasting and power cuts

Quick answer

The most common way to lose fish while you are away is not starvation. It is somebody feeding them. Healthy adult fish tolerate a period without food far better than they tolerate a tank fouled by uneaten food, and a well-meaning neighbour who thinks the fish look hungry can wreck the water chemistry in a couple of days.

The second most common way is a failure nobody was there to notice: a heater that stuck on or off, a filter that stopped, or a power cut. Those are the risks worth spending your preparation time on.

The tank you leave behind should be stable and boring. Nothing new, nothing recently changed, and nothing that depends on somebody making a judgement call.

• Most healthy adult community fish are safe for a short trip with no feeding at all.
• Never use a plaster-based holiday feeder block. It dissolves into the water and raises hardness while releasing far more food than the fish need.
• If someone is feeding, pre-portion every meal into separate containers. Do not leave a tub of food.
• Test any automatic feeder for a full week before you rely on it.
• A single dead fish in an unattended tank can trigger an ammonia spike that kills the rest.

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Do not leave a tub of fish food with a sitter.

The single most reliable way to kill an aquarium during a holiday is to give an untrained person an open container of food and a vague instruction. People feed to the fish's apparent enthusiasm, and fish always look enthusiastic. The uneaten remainder decomposes, ammonia rises in a tank whose filter is sized for normal loading, and the fish die of poisoning rather than hunger. Pre-portion each feeding into a separate labelled container or a weekly pill organiser, and hand over only those.
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Decide in 60 seconds

Why fasting is usually the safe option

Fish are ectotherms. They do not spend energy maintaining a body temperature, and their daily energy requirement is a fraction of a similarly sized mammal's. In the wild, food availability fluctuates and most species are adapted to periods with little or nothing.

A healthy, well-conditioned adult fish in a stable tank therefore comes to no harm over a short absence, and many keepers deliberately include a fasting day in their normal week.

What fish tolerate very badly is the consequence of overfeeding. Every particle of uneaten food decomposes into ammonia. The filter's bacterial population is sized for the waste it usually receives, and it cannot expand quickly. So a doubled food input produces ammonia that the filter cannot process, in a tank nobody is watching.

The asymmetry is the whole argument. Underfeeding a healthy adult fish for a week is a mild, reversible stress. Overfeeding it for a week can kill the entire tank.

That said, the exceptions are real and they matter. Fry and juveniles are growing rapidly and have very little reserve. Species that graze continuously on algae or biofilm need a food source present rather than meals. Some small, high-metabolism species and some specialist feeders do not tolerate fasting well. If your tank contains any of those, the plan has to be a person rather than a fast.

The feeding options, ranked

No feeding at all.

For a short absence with healthy adults, this is the safest option and it requires no equipment. Feed normally the day before you leave, do not overfeed as compensation, and feed normally when you return.

A sitter with pre-portioned meals.

The best option for longer trips. Portion each feed into a separate small container, a weekly pill organiser, or folded paper envelopes, labelled with the day. Give the sitter only those. This removes the judgement call entirely and it is the single most effective thing in this article.

Instruct the sitter to feed every other day rather than daily if that suits your stock, and tell them explicitly that if they are unsure whether they fed, they should skip it rather than repeat it.

An automatic feeder.

Reliable models work well, but they fail in specific ways. Humidity above a tank causes food to absorb moisture, swell and jam, so mount the feeder above the water line with a gap, use a lid that keeps the humid air out, and consider a small sachet of desiccant if the model allows.

Test it for at least a week before you travel, in position, with the food you actually use, and watch what it dispenses. Set it to less than your normal ration.

Check the batteries and replace them before the trip regardless of how much life they appear to have.

Do not use one for the first time on the day you leave.

Slow-release or plaster-based holiday blocks.

Avoid these. They are made of a binder that dissolves into the water, which raises hardness and can shift pH, and they release food continuously whether or not the fish want it. They combine the two problems this article is about: uncontrolled feeding and a change in water chemistry, in a tank with nobody watching.

Live plants and biofilm.

In a mature planted tank, grazing species find something to eat. This is a helpful background, not a plan.

Portion the whole trip out before you leave, one container per feed. The sitter's job then becomes emptying a container rather than judging a quantity.

Preparing the tank in the week before

Do the maintenance early, not on departure day.

Water change, filter rinse and glass clean about a week before you go, so that if anything is disturbed you are there to see it settle. A big water change or a filter service on the morning you leave is exactly the wrong time to find out something went wrong.

Test the water and write the numbers down.

Ammonia, nitrite, nitrate, pH and temperature. You want to leave knowing the baseline, and you want it recorded for comparison when you return.

Check the heater and the thermostat.

Watch the temperature over several days with a thermometer that is not the heater's own dial. Heaters fail on as well as off, and a stuck-on heater cooks a tank quickly. Consider two smaller heaters rather than one large one, so a single failure is survivable in either direction.

Check the filter.

Listen to it, watch the flow, and clean impellers and intakes so nothing clogs while you are away. Do not replace the biological media before a trip.

Put the lights on a timer and confirm the timer works. A light left on continuously produces an algae bloom and stresses the fish.

Leave headroom for evaporation and consider an automatic top-off if your system supports it. Evaporation concentrates everything dissolved in the tank and can expose heaters and pump intakes.

Check the lid.

A secure lid prevents jumping, which fish do more readily when disturbed or when water quality changes. It also reduces evaporation.

Remove anything that is likely to die.

A fish that is clearly failing should be dealt with before you leave, humanely and on advice if you are unsure. A dead fish decomposing in an unattended tank is a serious ammonia source, and in a small tank it can kill everything else.

Do not add new fish, plants or decor in the weeks before a trip, and do not rescape. New arrivals bring disease and disturbance, and both take time to show.

Do not start any medication before you travel.

Power cuts and equipment failure

This is where most catastrophic losses come from.

Without a filter, the biological bacteria begin to suffer from lack of oxygenated flow, and without a pump or airstone the surface stops exchanging gas. In a warm room, dissolved oxygen falls quickly because warm water holds less of it and the fish are metabolically active.

A battery-operated air pump is inexpensive and is the single most useful contingency item. Some switch on automatically when mains power is lost. Leave one with the sitter with instructions and spare batteries.

Insulation buys time in winter. Blankets or polystyrene around a tank slow heat loss substantially. In summer the risk reverses and a lid left partly open with a fan helps, though only if someone is there.

Tell a neighbour or the sitter what to do. Unplug and replug the filter when power returns. Check the heater is working. Do not feed after a power failure until the filter has been running again for a while.

Surge protection matters when power returns, because that is when equipment fails.

Leave a note on the tank with your number, the vet or shop you use, and the basic instructions, so that whoever is there does not have to find your message.

Instructions to leave for a sitter

Write them down. Do not rely on a conversation.

State what to feed, which container, on which days, and at roughly what time.

State clearly what not to do. Do not add extra food. Do not change water. Do not clean the filter. Do not add anything to the tank. Do not adjust the heater or the lights.

Include a photograph of the tank at the normal water line, so an unusual drop is obvious.

Include what to check each visit: are all the fish visible and swimming normally, is the filter running, is the light on when it should be, is the temperature reading normal, is the water clear.

Include what to do if a fish dies: remove it with a net, do not feed that day, and contact you.

Include what to do if the power has been off: what to plug back in and in what order.

Include your contact details, an alternative contact, and the name and number of a shop or aquatic vet who knows the tank.

Use the food the fish already eat. A holiday is not the time to introduce a different pellet, and a sitter cannot judge whether an unfamiliar food is being taken.

Coming home

Do not feed a double portion to make up for it. Feed normally, or slightly less than normal on the first day.

Test the water before you do anything else and compare with the numbers you recorded. Ammonia or nitrite above zero means a partial water change, not a full one, and an investigation into why.

Look at the fish carefully. Faded colour, clamped fins, gasping at the surface or hiding all point at a water quality event in your absence.

Check the water level, the heater temperature, the filter flow and the timer.

Look for uneaten food in the substrate, which tells you the feeding plan was too generous, and siphon it out.

Do a water change once you know where you stand, sized according to the test results rather than to habit.

What never to do

Do not use plaster-based holiday feeder blocks.

Do not leave an open container of food with anyone.

Do not use an automatic feeder you have not tested in position for a week.

Do not do a major water change, filter service or rescape immediately before leaving.

Do not add new fish or plants shortly before a trip.

Do not start a course of medication and leave.

Do not leave a visibly failing fish in the tank and hope.

Do not turn the lights off entirely for a planted tank, or leave them on continuously for any tank.

Do not overfeed on your return.

Do not assume the sitter will interpret a vague instruction the way you meant it.

When to get help

Ask an aquatic specialist or an experienced keeper before a long absence if you keep fry, a breeding project, a reef or marine system, koi of value, or any fish currently under treatment.

Get advice urgently on your return if the water tests show ammonia or nitrite, if fish are gasping at the surface, if several have died, or if the tank is cloudy and smells.

Contact your sitter's fallback contact rather than waiting if they report a filter that has stopped, a heater reading wrongly, or a fish behaving abnormally, because these are far easier to fix in the first day than the third.

A successful trip looks like no change at all: the same water line, the same fish, the same test results as the day you left.

Have ready the tank volume, the stocking list, the test results from before and after the trip, the feeding arrangement you used, and what the sitter observed. If a loss has occurred, keeping the body refrigerated rather than frozen preserves it for examination if that becomes worthwhile.