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Cracked fish tank: emergency steps, leak repair and when to replace

Act within the hour

If your fish tank is cracked or leaking, make the electrics safe first, then lower the water below the leak and move your fish into clean buckets with their filter media. A small chip can wait, but a crack in the glass or a failing bottom seam can give way without warning.

Once everyone is safe, you can work out whether it's a chip, a crack or a seam leak. Seam leaks can often be fixed with aquarium-safe silicone. A cracked pane, especially the bottom, usually means a new tank.

How to identify it

  • A chip: a small flake missing from an edge or corner, with no line running into the glass. It doesn't leak. Watch it, but it's rarely urgent.
  • A crack: a line running into or across the glass. You can often feel it with a fingernail on one side. Cracks can grow, especially with temperature changes or pressure.
  • A seam leak: water seeping where two panes meet, through the silicone (the rubbery sealant in the corners). You may see bubbles, gaps or peeling silicone on the inside bead.
  • Not a leak at all: drips from a hang-on filter, airline, a lid gap or condensation running down the outside. These sit above the waterline.

Finding the leak. Dry the outside of the tank and the stand completely. Run a dry paper towel along each seam and the bottom edge, then watch for 15 to 20 minutes. Water travels, so the wet spot on the floor is not always under the leak. Check the top of the stand, under the tank's plastic rim if it has one, and the back corners.

When it's a problem

  • Normal: a dry chip on a top edge, small scratches, or condensation.
  • Early problem: a slow weep at a corner seam, a small crack high on a side pane, or silicone that's peeling but not leaking.
  • Serious: any crack below the waterline, steady drips, or water pooling on the stand.
  • Emergency: a crack in the bottom pane, a crack that's growing while you watch, a bowing pane, a creaking sound, or a seam that's spraying or running. Assume the tank could fail completely and act now.

What to do right now

  1. Stay safe with electricity. If water has reached a plug, socket or power strip, don't touch it. Switch off that circuit at the breaker or fuse box first. Never stand in water to unplug something.
  2. Unplug the heater, filter and lights once it's safe. Let the heater cool for a few minutes before lifting it out, so the glass doesn't crack.
  3. Lower the water below the crack or leak. Use a siphon or jug and fill clean buckets with tank water. This water is precious: it's what your fish are used to.
  4. Protect the floor. Put towels down, move rugs, electronics and books, and put a tray under a slow drip.
  5. Move the fish into the buckets of tank water. Use clean buckets that have never had soap or chemicals in them.
  6. Move the filter media too. Put the sponge or bio media into a bucket with the fish so the good bacteria stay alive. Run the filter on the bucket if you can, or add an air stone.
  7. Keep them warm. Put the heater in a large bucket once it's fully submerged, or cover the buckets with a towel. Don't feed for a day.
  8. If the bottom is cracked or failing, empty the tank fully. Don't leave water in it overnight.

What to do next

Set up a temporary home. A storage tub, a spare tank or even a clean bathtub can hold fish for a few days. Run the old filter (or its media in a sponge filter), keep the heater in, and test ammonia daily. Do small water changes with dechlorinated, temperature-matched water if ammonia rises. See the ammonia guide.

Decide: reseal or replace.

  • Reseal if the glass is whole and one seam is leaking.
  • Replace if any pane is cracked (especially the bottom), the tank is large and old, several seams are failing, or the bottom glass is tempered. Tempered glass can shatter all at once.
  • Acrylic tanks can sometimes be repaired with special solvent cement. Ask a shop that builds acrylic tanks.
  • Under warranty? Take photos and keep your receipt before you take anything apart.

How to reseal a seam (glass tanks).

  1. Empty the tank completely and let it dry.
  2. Cut away the old inside silicone along the leaking seam with a new razor blade. Leave the thin layer between the panes alone; it holds the tank together.
  3. Remove every trace of old silicone. Wipe with acetone or rubbing alcohol on a lint-free cloth, then let it dry. New silicone won't bond well to old silicone.
  4. Apply a smooth, even bead of aquarium-safe silicone (about 6 mm, or 1/4 inch) along the seam, with no gaps or bubbles. Masking tape on each side gives a neat line.
  5. Let it cure. Follow the tube. Many say 24 to 48 hours, but thick beads and cold or damp rooms take longer. Many hobbyists wait about a week before filling. Air the room well.
  6. Water test before fish go back. Fill a few centimetres (inches) at a time, checking each seam as it rises. Leave it full for at least a day somewhere a leak won't do damage, like a garage or a bathtub.

Use only aquarium-safe silicone. Bathroom and kitchen sealants often contain mould killers (fungicides) that can poison fish. "100% silicone" on the label isn't always enough. Look for one sold specifically for aquariums.

Fish health afterwards. Moving is stressful. Watch for ich and fin rot over the next two weeks, and keep the water clean. If fish were in a bucket without filter media for a long time, your tank may need to cycle again.

Products that help

  • Aquarium-safe silicone sealant: for resealing seams. Must be free of mould and mildew killers.
  • Razor blade scraper and spare blades: for removing old silicone from glass (never acrylic).
  • Acetone or rubbing alcohol: cleans away old silicone and grease before resealing.
  • Clean buckets or a large storage tub: temporary homes. Keep them just for fish.
  • Battery air pump: keeps oxygen in buckets if power is off or you're moving fish.
  • GFCI (ground fault) outlet or power strip: cuts power if current leaks into water. In some countries it's called an RCD.
  • Leak or water alarm: a cheap sensor on the floor beside the stand that beeps when it gets wet.
  • Spirit level and plastic shims: for checking and correcting the stand.
  • Keep on hand: towels, a siphon, a spare heater, dechlorinator, and a sponge filter.

How to prevent it

  • Use a level, strong stand made for the weight. Water weighs 1 kg per litre (about 8.3 lb per US gallon). A 200-litre (53-gallon) tank weighs well over 200 kg (440 lb) with gravel and rock.
  • Check level front-to-back and side-to-side. Correct it with plastic shims, not wood, which can compress or rot.
  • Avoid point loads. A grain of gravel, a screw head or a warped stand under the glass can crack the bottom. Rimless tanks often need a foam mat; framed tanks usually sit on their frame. Follow the maker's instructions.
  • Never move a tank with water or gravel in it. Twisting a filled tank strains the seams.
  • Don't drop rocks. Place heavy rock gently. Many keepers set big rocks on a sheet of plastic "egg crate" grid on the bottom, which spreads the weight and stops fish digging under them.
  • Keep the heater off the glass and unplug it before water changes so it never runs dry.
  • Use drip loops: let each cord dip below the socket so water drips off the loop instead of reaching the plug.
  • Inspect seams once a month for peeling or bubbles.

Why it happened

Most cracks come from an uneven stand, a point load under the glass, a knock, a dropped rock, or a heater touching the glass. Seam leaks come from age, poor original sealing, or a tank being moved or twisted while full. Silicone also slowly breaks down over many years.

When to get help

Get help if the tank is large (over about 200 litres, or 50 gallons), if a bottom pane is cracked, if water has reached wiring or soaked into floors or ceilings below, or if you're not confident resealing. An aquarium shop or tank builder can reseal or replace it. For electrical damage, call an electrician. For water damage, check your home insurance.

In the game

Multiple Tank Syndrome doesn't have a cracked tank card yet. The closest is the "City power failure" disaster, which also sends you hunting for a battery air pump.

Sources

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