Aquarium leak, crack or burst tank: the first hour

Quick answer

A small container of the fish's own water is where a fish ends up when its tank fails. That is survivable for hours, not for days.

An aquarium that is leaking, cracked or seeping is two emergencies at once: an electrical hazard in your room, and a life support system failing around your fish.

Deal with them in that order. Power off first, fish second, water damage third, and the tank itself last, because a tank that has begun to fail cannot be saved by anything you do while it is still full.

• Switch the power off at the socket before you touch anything wet. Heaters, filters and pumps are mains equipment sitting in water.
• Never lift or move a tank with water in it. A stressed pane fails when the load shifts.
• Lower the water into clean buckets and keep that water. It holds your chemistry and your bacteria.
• Move the fish into a covered bucket of that old water with air, and keep the filter media wet in tank water.
• Most suspected leaks are not the tank. Condensation, a back-siphoning filter and a loose hose are far more common than a cracked pane.

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Water and mains electricity are the reason this is an emergency for you, not only for the fish.

Before you mop, bail, reach behind the tank or touch a heater, switch off the supply at the socket, and ideally at the consumer unit or breaker for that circuit. Water travelling along cables and into a socket or extension lead is the mechanism, and a heater exposed to air while still powered can also shatter, which is how a small leak becomes broken glass in the water.
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Decide in 60 seconds

Why aquariums fail, and why they fail suddenly

Glass does not wear out gradually and then warn you. It accumulates stress and then releases it all at once, which is why owners describe a tank that was "completely fine yesterday".

The stand and the floor.

The base pane of a glass tank is designed to be supported evenly across its whole area. Anything that turns even support into point loading concentrates stress at one spot.

A single grain of gravel trapped under the base, a stand that has warped slightly, a plinth that is not quite flat, or a tank set on carpet that compresses unevenly all do the same thing. The crack usually appears near that point and then runs.

This is also why a levelling check is not optional and why many manufacturers specify a foam or polystyrene mat under the base. Where the manufacturer specifies one, its job is to distribute load, not to cushion.

Weight.

Fresh water weighs approximately one kilogram per litre. Add substrate, rock and the tank itself and the total mass sitting on a footprint the size of a small table becomes considerable. In an upstairs room, an old timber floor, or a converted flat, that load matters, and a tank should sit across joists rather than along a single one where possible.

Impact damage that shows up later.

A chip on an edge, made when the tank was delivered or when a rock was dropped, is a stress riser. It can sit there for years and then propagate when the temperature changes or the stand settles.

This is the reason to lower hard ornaments and rocks into a tank rather than dropping them, and to build hardscape on the base only after checking that nothing is trapped underneath.

Silicone ageing.

The seams are structural in a glass tank. Silicone loses adhesion over years, faster if the tank has stood in direct sunlight, and faster if the original surface preparation was poor. Seam failure usually starts as a weep at a corner rather than a sudden burst.

Thermal stress.

Pouring hot water into a cold tank, or cold water onto hot glass, creates uneven expansion. It is a real cause of cracking during water changes, especially in winter when tap water is very cold.

Acrylic behaves differently.

Acrylic tanks craze rather than crack cleanly, scratch easily, and can often be repaired by a competent specialist. They also flex, which means the stand requirements are different and usually stricter. If you have acrylic, do not assume the glass advice transfers.

What is actually leaking, because it usually is not the tank

Lower hard ornaments and rocks in, do not drop them. A chipped edge on the base pane is a crack waiting for a temperature change.

Before you condemn a tank, work through the far more common causes. Owners routinely strip and rebuild a perfectly sound aquarium because of a loose hose clip.

Condensation.

A cold room and a heated tank produce running condensation down the outside of the glass, which pools at the base and looks exactly like a seam leak. Dry the outside completely, lay kitchen paper around the base, and look again in an hour.

Canister filter connections.

Loose taps, perished O-rings, a lid not seated evenly, and hoses that have hardened with age. A canister leak often appears at the cabinet floor, well away from the tank, and is the single most common source.

Back-siphoning.

When a pump stops, water can siphon backwards down the outlet or through an air line that runs below the water level, emptying the tank onto the floor. Check that air lines rise above the water surface before they descend to the pump, and that outlets sit high enough to break the siphon.

Sump overflow.

In a sump system, the volume that drains back when the return pump stops must fit in the sump. If it does not, the sump floods every time the power blips.

Heater seals and cracked housings.

A heater with a hairline crack in its glass can leak and is also an electrical hazard. Any heater that has been run dry should be replaced, not tested.

Overflowing during a water change.

Obvious in hindsight, and worth ruling out before panicking.

Evaporation misread as a leak.

A dropping water level with a dry floor is evaporation. Open-top tanks in a heated room lose a surprising amount, and it accelerates in winter when central heating dries the air.

The test.

Dry everything, mark the water level with a strip of tape, and turn off equipment one item at a time, giving each an hour. If the level drops with everything off and the floor stays dry, it is evaporation. If the floor is wet only when a specific device runs, that device is the leak.

The emergency sequence

1. Power off.

At the socket, and at the breaker if water has reached cables or an extension lead. Do not skip this because the leak looks small.

2. Do not move the tank.

A tank containing water must never be lifted, slid, tilted or levelled. The water inside moves as a mass and the stress goes straight into the seams and base. Tanks that were merely weeping have failed completely at this moment.

3. Get containers ready.

Clean buckets that have never held detergent, bleach or plant food. Food-grade buckets are ideal. Household cleaning buckets retain residue that will kill fish.

4. Lower the water into those buckets, and keep it.

Siphon, do not bail with a jug if you can avoid it, because reaching into the tank moves it. Take the level below the crack or seam and stop there if the tank is stable, or drain fully if the base is involved.

That water is worth keeping. It is at the right temperature, the right hardness and the right pH for your fish, and it contains no chlorine. Sudden changes in those parameters are a bigger threat to a fish in the next hour than a slightly cloudy bucket.

5. Move the fish.

Net them calmly into a bucket of their own water. Do not chase, and do not empty the tank down to a puddle and then catch them, because a fish in a few centimetres of water is easily injured and severely stressed.

6. Rescue the filter media.

Take the sponges, ceramic media and filter floss out and submerge them in a container of tank water. They hold the nitrifying bacteria that process ammonia, and that colony dies in air or in chlorinated tap water within a short time.

This is the single most valuable thing you will save. Substrate, plants and decor can all be replaced quickly. A mature bacterial colony takes weeks to grow.

7. Deal with the water damage.

Lift anything absorbent off the floor, get air moving, and if the tank was upstairs or in a flat, check the ceiling below. Many household insurance policies treat aquarium leaks differently from plumbing leaks, and it is worth knowing your position before you need it rather than after.

Keeping fish alive in a bucket

As the water level falls, the heater and the filter intake become exposed. Switch both off before the level reaches them, and never let a heater run in air.

Holding containers are survivable, but everything that keeps a fish alive in an aquarium now has to be done by you.

Oxygen.

The surface is where gas exchange happens, and a bucket has a small surface relative to its depth. An air pump with an air stone is the reliable answer, and a battery-powered air pump is the single most useful piece of emergency equipment an aquarist can own.

Without one, keep the fish density low, use wider containers rather than deeper ones, and disturb the surface periodically. Warm water holds less oxygen than cold, so a heated bucket needs more air, not less.

Ammonia.

With no filter, ammonia from the fish accumulates immediately, and ammonia is more toxic at higher pH and higher temperature. Two things control it.

Do not feed. A healthy fish tolerates several days without food far better than it tolerates ammonia. Fasting cuts waste production sharply.

Change part of the water daily, using dechlorinated water matched for temperature. If you have an ammonia-binding conditioner, this is what it is for.

Temperature.

Tropical fish need their temperature held. A heater can be used in a bucket only if it is fully submerged to its minimum mark and the container is stable; many are not designed for shallow containers and will shatter or overheat.

Safer alternatives are keeping the container in the warmest room, wrapping the outside with towels or a blanket, and floating a sealed bottle of warm water to nudge the temperature. Change temperature slowly. A rapid swing is more dangerous than a slightly wrong steady value.

Coldwater species have the opposite problem. Goldfish in a warm room in summer suffer, and their oxygen demand rises as the water warms.

A lid.

Many fish jump when stressed, and bettas, danios, hatchetfish and most catfish are notable for it. Cover the container with a lid that lets air in, such as a towel or a piece of mesh, not a sealed one.

Darkness and quiet.

A covered container in a low-traffic room reduces stress considerably. Resist the urge to keep checking.

Aggression.

Fish that coexisted in a decorated tank may not coexist in a bare bucket, because there is nowhere to break line of sight. Separate known aggressive individuals, and add a piece of clean plastic pipe or a plant for cover if you can.

Repair, or replace

Be honest about this, because a second failure with a repaired tank usually happens when nobody is home.

Base pane damage: replace the tank.

There is no reliable home repair for a cracked base. The entire water column sits on it.

Seam weep: repairable, but not by patching.

Silicone does not bond to cured silicone. A genuine seam repair means fully draining and drying the tank, cutting out the old internal silicone bead with a blade, cleaning the glass thoroughly, and laying a fresh bead. Skimming new sealant over the old one produces a repair that looks fine and holds until it does not.

Use aquarium-grade silicone only. Bathroom and kitchen sealants contain fungicides and mould inhibitors that leach into water. Let it cure for the full time the manufacturer states, then leak-test with water outside or somewhere the failure does not matter, for at least a day.

Cracked side or front pane.

Occasionally patched with a glass plate siliconed over the crack on a small tank, and reasonably common practice among experienced keepers. It is not equivalent to an intact pane, and it should never be done on a large or deep tank.

Getting the tank back.

When you refill, you are also rebuilding the biological filter. If you saved the media wet and it was only out of water briefly, much of the colony survives and the tank may re-establish quickly.

Either way, test ammonia and nitrite daily for the first two weeks, feed lightly, and add fish back gradually rather than all at once. A tank that has been drained and refilled behaves like a partly cycled tank, not a mature one.

The routine that prevents the next one

What never to do

Do not reach into or behind a leaking tank before switching off the power.

Do not attempt to move, lift or reposition a tank with water in it. This includes sliding it a few centimetres to look behind, and includes "just tilting it" to get a mat underneath.

Do not top up a leaking tank to buy time. You are adding load to a failing structure.

Do not rinse filter media in tap water. Chlorine and chloramine kill the bacteria you are trying to save.

Do not use household buckets that have held detergent, bleach or garden chemicals. Residues at concentrations you cannot smell will kill fish.

Do not use bathroom silicone, aquarium-safe adhesive tape, or any product that does not explicitly say it is intended for aquarium use in contact with water.

Do not feed fish in a bucket for the first day or two. You are choosing between a hungry fish and an ammonia-poisoned one.

A fish in a holding container will often sit still on the bottom. Stillness on its own is not alarming in the first hours; fast or laboured gill movement is.

When to get help

Contact an aquatic specialist or an experienced local fishkeeper the same day if you have a crack in any pane, a base pane involved at all, or a seam that has separated over more than a short length. A shop that builds or repairs tanks can tell you in a minute whether a tank is worth saving.

Contact an aquatic or exotics veterinarian if fish are gasping, lying on their sides, bleeding, or showing red streaks in the fins after a failure, and particularly if several are affected at once. A sudden temperature or chemistry change during an emergency move does real physiological damage, and gill damage from an ammonia spike develops over days rather than minutes.

Contact an electrician if water has entered a socket, an extension lead or a consumer unit. Drying it out and hoping is not a plan.

For any professional you speak to, have this ready: the tank volume and dimensions, its age, what it stands on, the water parameters before the failure, where exactly the water appeared, what equipment was running, and what you have already done. That set of facts identifies the cause faster than any inspection of the empty tank.