Key takeaways
- Tightening the clamps is the wrong first move and often warps the head so a seal that was recoverable no longer is.
- White kitchen paper laid under every joint for an hour finds a weep that no amount of staring at a dark cabinet floor will.
- The head gasket and the hose taps between them account for the large majority of canister leaks, and both are cheap parts.
- A sit test off the tank, filled and sealed on dry paper for two hours, proves whether the housing or the plumbing is at fault.
- A cracked canister body is not a repair job, because sealant on a pressure joint fails without warning and usually at night.
The instinct on finding water under a cabinet is to reach in and tighten the head clamps. Resist it. Over-clamping bows the plastic rim, and a gasket that was merely dirty becomes a gasket that will never seat properly again.
A canister runs a closed loop below the water line, so it leaks under a meter or so of head pressure, not much. That is why most canister leaks are slow weeps rather than floods, and why they are so hard to see.
The whole job is about making the filter reveal itself. Nine times out of ten it does, in under two hours, without a single part being bought first.
Unplug the canister at the wall before touching it, and unplug anything else standing in the wet area. Water on a cabinet floor next to a power strip is the single most common way an aquarium becomes an electrical incident. If the strip itself is wet, cut power at the consumer unit before you reach in.
Rule out the things that look like leaks
Condensation is the first impostor. A canister running at 24C (75F) in a room at 18C (64F) will sweat, and the film covers the whole body evenly rather than tracking from a point.
The second is splash. Check the tank rim above the cabinet, any hose that runs over an edge, and the spray bar, because water wicking down a hose exterior lands in exactly the same place a head gasket leak does.
The third is siphon overflow during maintenance, which people forget about within a day. If the damp appeared the morning after a service and has not grown since, it may simply be the water you spilled.
Dry it down and lay the paper
Take everything out of the cabinet, wipe every surface of the canister and the floor, and let it stand ten minutes to finish drying. Do not skip this. You cannot read new water against old water.
Then tear white kitchen paper into strips about 3 cm wide and place one under each candidate joint: beneath the head seam, wrapped loosely round each hose tap, under each barb where hose meets fitting, and one flat under the base.
Leave it an hour. Paper turns translucent at the first drop, so even a weep of a few milliliters shows as a clear grey patch with a definite centre.
- Head seam, all the way round including the back
- Both hose taps or the quick disconnect block
- Each barb and hose clamp, inlet and outlet
- The priming button or pump housing, if your model has one
- Flat under the base, to catch anything running down the body
Read the pattern, not the puddle
Water runs down and sideways along moldings before it drops, so the wet paper is downstream of the fault, never upstream. Follow the trail up the body until it stops.
A leak at the head seam typically shows as a damp arc on one side rather than a ring. That arc points straight at where the gasket is not compressing, which is usually where it was pinched or twisted during the last service.
The four usual suspects, in order of odds
| Suspect | How it presents | Fix | Cost |
|---|---|---|---|
| Head gasket seated wrong | Damp arc on one side of the seam | Clean the groove, regrease, reseat | $0 |
| Head gasket worn out | Weep returns after reseating | New sealing ring | $8 to $18 |
| Tap or valve block O-rings | Drip from the disconnect, worse after removal | New O-ring set, greased | $5 to $15 |
| Hose to barb joint | Damp collar at the hose end | Trim 10 mm off the hose, refit, add a clip | $0 to $3 |
All four of those are parts you are meant to change yourself, which is the useful thing about canister leaks. Cracked housings exist but are rare, and almost always follow either over-clamping or a canister left somewhere that froze. Check the base corners and the rim with a flashlight held at a low angle if the four above come up clean.
The sit test
If the paper does not settle it, take the filter off the tank. Close both taps, disconnect, carry it to a bathtub or a large tray, and open it.
Refill the body with tap water to the normal level, refit the head with the gasket exactly as you found it, and clamp it down. Stand the whole unit on a double thickness of dry paper towel in the tray and leave it two hours, untouched and unplugged.
Wet paper after two hours means the housing or the head seal is at fault. Dry paper means the canister is sound and the leak is in the plumbing: taps, hoses, barbs, or the connection at the tank.
Do not pressurize a canister with a bicycle pump or an air compressor. These are low head vessels, not pressure vessels, and a few psi more than they were designed for will split a rim or fire the head across the room.
Fixing the head gasket
Lift the seal out of its groove completely rather than working round it in place. Wipe the groove with a lint free cloth and run a fingernail round it, because a single grain of sand or a strand of filter floss is enough to hold the seal open.
Wipe the seal itself, inspect it, then apply a thin film of pure silicone grease and press it back in evenly without stretching any section. Refit the head with steady pressure on opposite clamps in turn, the way you would tighten wheel nuts.
If it weeps again after that, the seal is finished rather than dirty. Knowing when a seal is past regreasing saves you doing this twice, and a replacement sealing ring for an Eheim Classic or a Fluval 07 series head is around $8 to $18.
Taps, valve blocks and hose ends
Quick disconnect blocks carry small O-rings that dry out faster than the head gasket because they are exposed to air every time you service the filter. They are the most common leak on any canister more than three years old.
Pull the block apart, replace the O-rings as a set rather than singly, and grease them. Do not reuse one that has flattened on one side.
Hose ends harden and go oval where they have gripped a barb for years. Trimming 10 mm off the end with a sharp blade gives you fresh, round, flexible material, and it costs nothing. Dip the end in hot water for thirty seconds first so it slides on fully.
Finding the drip that only showed up overnight
A 200 liter (53 US gallon) planted tank with an Eheim Classic 2213 developed a damp patch about the size of a coaster, front left of the cabinet, roughly a week after a media change.
Paper strips showed dry taps and a wet arc on the head seam at the front left. Lifting the head revealed the green sealing ring sitting proud of its groove across about 40 mm, where it had been pushed in twisted and never dropped home.
Cleaning the groove, greasing the ring and reseating it fixed it for nothing. The filter has run three years since. Had the owner tightened the clamps instead, the rim would have taken a set and the head would have needed replacing at around $70.
What not to do
Silicone sealant does not belong on a serviceable joint. It will hold for a few months, fail without warning, and glue a $12 gasket groove into a $70 replacement head.
Petroleum jelly does not belong anywhere near these seals either. It swells several of the rubbers used in aquarium equipment and turns a good seal soft and oversized within months.
Do not run a leaking canister on a towel while you wait for parts. Take it off the tank and put an air stone in, because a slow weep at 2 a.m. becomes a siphon if a joint lets go completely.
Getting it back online
Reconnect with the taps closed, open the outlet first, then the inlet, and watch the head fill. Run it for twenty minutes with fresh dry paper under every joint before you put the cabinet contents back.
Check again at twenty four hours and at one week. Seals that have been disturbed sometimes settle and weep on the second day rather than the first, which is why one dry hour is not a result.
If you have been through all of this and it still leaks, the honest answer is a new head. Work through the wider fault path for aquarium equipment to be sure the canister is genuinely the source, and keep a set of the seals your model uses in a drawer so the next one takes twenty minutes instead of a week.
Questions readers ask about this
Intermittent leaks usually track temperature or servicing. Plastic expands and contracts with room temperature, so a marginal seal can hold at 22C (72F) and weep at 16C (61F) overnight. The other pattern is post service: the leak appears within a day or two of the head coming off, which points at the gasket seating rather than the gasket itself.
Not on a gasket joint. PTFE tape works on threaded fittings only, and silicone sealant on a serviceable seal groove will fail unpredictably and ruin the groove for the correct part. The joints on a canister are designed to seal by compressing rubber, so the repair is always cleaning, greasing or replacing that rubber.
A weep at a head seam typically loses 50 to 500 ml a day, which evaporates fast enough that you find damp rather than a puddle. The danger is different: if a hose or a tap fails completely while the canister sits below the water line, it will siphon until the tank drops to the intake, which can be half the volume.
Only if they are stiff, oval at the ends, or narrowed inside by biofilm. Otherwise trim 10 mm off each end and refit. A full set of 12/16 mm or 16/22 mm hose runs around $15 to $30, and it is worth doing at five years or so, when the material has usually hardened enough to stop gripping the barbs properly.
The water itself is a minor problem. The electrical risk is not, because power strips and plugs usually sit on the same cabinet floor the water lands on. Unplug everything in the wet area before reaching in, keep the strip mounted above the level of the equipment, and make sure the circuit is protected by an RCD or GFCI.
