Key takeaways
- Almost every equipment fault reports itself as one of five symptoms, and each symptom has a cheapest likely cause that should be ruled out before any part is bought.
- An RCD or GFCI trips on roughly 5 milliamps of leakage, which is the protection that matters near water, while the breaker in your consumer unit is only protecting the cable.
- Sound changes before performance does, so a pump that has started grinding is giving you weeks of warning rather than none.
- Sealed mains units such as potted LED drivers, molded plugs and bonded heater tubes are replacement items, not repair items.
- A drawer holding a spare impeller, silicone grease, spare seals, a backup heater and a battery air pump costs around $60 to $90 and has saved more tanks than any diagnostic skill.
A customer called about a 240 liter (63 US gallon) planted tank whose canister had gone silent overnight. He had already ordered a replacement pump head, around $90, before he opened the cabinet door. The actual fault was a chipped impeller blade, a part costing about $14, and the swap took eleven minutes.
Buying the part before reading the symptom is the most expensive habit in this hobby. Aquarium equipment fails in a small number of recognizable ways, and each one narrows the field fast if you let it.
This page is the map for the whole repair section here. Work down it in order, because the order is doing most of the work.
Kill the power before anything else
Before a hand goes into the water or behind a cabinet, pull the plug out of the socket. Not the switch on the power strip. The plug, where you can see it sitting on the floor.
Everything mains powered near an aquarium belongs behind a residual current device: an RCD in the UK and Europe, a GFCI outlet or breaker in North America. These trip on roughly 5 milliamps of current leaking to earth, far below anything a 15 or 20 amp breaker will notice. The breaker protects the cable inside your wall, not the person standing in front of the tank.
If your tank is not on one, that is the first job on the list. A GFCI outlet costs around $20 to $35 in the US and a plug in RCD adapter is similar money in the UK, though fitting a new outlet is electrician work in most places. The Electrical Safety Foundation International publishes plain guidance on where these devices are required.
Never open a sealed mains unit. Potted LED drivers, molded plugs, heaters with a bonded glass or titanium tube, and pumps with an encapsulated stator are all built to be replaced rather than serviced. Cutting one open removes the insulation standing between mains voltage and your tank water.
Five symptoms cover almost everything
Nearly every fault I get called about lands in one of five buckets. Find yours here, then read the matching section below.
| Symptom | Most likely cause | Typical fix cost |
|---|---|---|
| No power at all | Fuse, dead socket, or a failed thermostat contact | $0 to $15 |
| Breaker or RCD trips | Water tracking into a plug, or a cracked heater | $0 to $60 |
| New noise | Impeller, shaft, or trapped air | $8 to $35 |
| Reduced output | Blocked intake, packed media, stiff impeller | $0 to $20 |
| Water outside the tank | Gasket seated wrong, or a tired hose fitting | $0 to $25 |
Nothing turns on
Test the socket before you suspect the equipment. Plug a lamp into the same outlet, then try the aquarium unit in a different outlet you know is live.
If it is dead in both, look for a fuse. UK plugs carry a cartridge fuse, typically 3 amp for lights and small pumps and 5 amp for larger gear, and these fail more often than people expect. Plenty of LED drivers also hide an inline fuse on the low voltage side of the transformer.
A heater that never warms is a different animal, because the element can be perfectly sound while the thermostat contact has failed open. If the indicator never lights at any dial setting, work through what a failed heater thermostat looks like on the bench before writing the unit off.
The breaker or RCD keeps tripping
These are two different messages. An RCD trip means current is leaking to earth, and around an aquarium that path is water. A conventional breaker trip means too much current is being drawn, which points at a dead short or a seized motor.
Unplug everything on that circuit, reset the device, then plug items back in one at a time with two or three minutes between each. The item that trips it is the item that is wet inside. In my experience it is a heater with a hairline crack, or a plug with crystallized crust bridging the pins, more often than anything else.
Work through the full sequence for a tank circuit that keeps cutting out if the one item at a time test does not find it. Do not fit a larger breaker, under any circumstances.
It sounds different than it did last week
Sound changes before performance does, which makes it the best early warning you have. A pump that has started to grind is describing a shaft that will fail in weeks rather than years.
Four sounds cover most of it: a steady hum, a rattle, a grind, and a rising whine. Each one points at a different part, and telling them apart by ear saves you stripping a pump that was never faulty.
One quick win first. Air trapped in a canister head rattles like gravel in a tin, and it usually clears if you tilt the whole unit about forty five degrees and rock it twice.
Output has dropped
Measure it before you diagnose it. Hold a 1 liter jug under the outflow and time the fill: nine seconds means roughly 400 liters per hour (106 US gallons per hour), which is close enough to normal for an Eheim Classic 2213 rated at 440 that you should look elsewhere.
If the number really has fallen, work up in cost order rather than pulling the pump first.
- Clear the intake strainer and look for a snail shell or a leaf jammed inside it
- Squeeze the hoses along their length, feeling for the soft narrowed section where biofilm has built up
- Lift the media baskets and check the top layer of floss, which always clogs before the ceramic does
- Pull the impeller, clean the well with a cotton bud, and spin the shaft between finger and thumb
Most flow complaints stop at the first or second item on that list. A monthly five minute intake clean prevents the majority of them.
There is water where there should not be
Dry every surface with a cloth, then lay strips of white kitchen paper under each joint and leave it for an hour. Paper shows a slow weep that your eye will miss on a dark cabinet floor.
On a canister, the head gasket and the hose taps account for the large majority of leaks. The method for narrowing it down is in tracing a canister leak joint by joint, and the seal is usually the repair rather than the housing.
Rubber has a service life. If a seal has gone hard, flat in cross section, or shiny along the contact face, replacing it rather than regreasing it is the honest answer.
Two faults that are not faults
Low water level wrecks more diagnoses than any other single thing. Let a tank drop 2 cm below the return and the filter starts pulling air, the outlet rattles, and the heater sits in a warmer layer than its own sensor expects.
The second is the timer. Before condemning a light or a CO2 solenoid, plug it straight into a wall socket and see whether it behaves normally. Mechanical timers with worn pins fail open or closed at random and cost around $8 to replace.
Cheapest cause first, with one exception
The order in every section above is deliberate: free checks, then cheap parts, then expensive parts, then replacement. It feels slow, and it is still faster than guessing.
The exception is age. If a unit is out of warranty, past five years of continuous service, and needs a part costing more than half the price of a new one, stop there. Pumps in particular rarely justify a second major part.
Write the date on a strip of masking tape and stick it to every impeller, seal and heater you fit. Two years later that tape answers the question you cannot otherwise answer: is this part old, or is it new and wrong?
When to stop and call an electrician
Some faults are not yours to fix, and recognizing them is part of the skill.
- An RCD that still trips with every aquarium item unplugged. The fault is in the building wiring.
- Any scorch mark, melted plastic, or burnt smell at an outlet, plug or extension strip.
- A socket that feels warm to the back of your hand, or one a plug sits loosely in.
- Any repair that would mean opening a sealed mains enclosure.
- A tingle felt when you touch the water and the metal tank rim at the same time. Cut the power at the consumer unit and call someone the same day.
Keep these five things in a drawer
A spare impeller for your main pump, a tube of pure silicone grease, the set of seals your canister uses, a 100 watt backup heater, and a battery powered air pump. The whole kit runs about $60 to $90.
Every one of those items has saved a tank for someone I know, usually on a Sunday evening with the shops shut. Buy them while nothing is wrong.
Questions readers ask about this
Unplug everything on the circuit, reset the RCD or GFCI, then reconnect one item at a time leaving two or three minutes between each. The unit that trips the device is the faulty one. If the device still trips with nothing aquarium related plugged in, the problem is in the house wiring and needs an electrician.
Only if the part is cheap. An impeller or shaft at $8 to $20 is worth fitting to almost any pump. A replacement motor block at $60 on a pump that costs $110 new is not, because the bearings, seals and cable have all aged at the same rate and the next failure is usually months away.
Intake strainer, hoses, media, then impeller. That is strict cost order: the first two are free and take three minutes, the media costs a rinse, and the impeller is the only step that involves buying anything. Roughly two thirds of flow complaints stop at the strainer or a partly collapsed hose.
You can, and you should not. A standard 15 or 20 amp breaker only trips on current high enough to endanger the cable, which is hundreds of times more than the amount that stops a heart. A GFCI outlet or plug in RCD adapter costs around $20 to $35 and covers every device on the tank.
The light usually shows the thermostat is calling for heat, so the fault is downstream: a failed element, or a heater badly undersized for the room. Check the wattage first. A tank needs roughly 3 to 5 watts per US gallon in a room that sits 5C (9F) below the target temperature.
