The Fluval FX series is a superb canister filter, and like every canister filter it depends entirely on rubber to keep several litres of water inside a sealed vessel. When an FX starts weeping, it is nearly always one of three seals — and which one tells you a lot about what went wrong.
1. The lid o-ring — A20210
The large o-ring around the filter lid. Symptoms: water pooling around the base of the canister after a media change, or a slow seep that appears within hours of closing the unit up.
This is the seal most often damaged by the owner rather than by age. It sits in a groove that has to be clean, and a single strand of filter floss or a grain of gravel across it will hold the lid open by a fraction of a millimetre.
Replacement for Fluval A20210 FX O-Ring (Filter Lid) for FX4 / FX5 / FX6
2. The motor seal ring — A20211 and A20207
Seals the motor assembly to the canister body. Symptoms: water beneath the motor housing specifically, often with reduced flow because the pump is drawing air.
Unlike the lid seal, this one usually fails from age and heat rather than from handling.
Replacement for Fluval A20211 FX4 / FX5 / FX6 Filter Motor Seal Ring · A20207 Motor Seal Ring
3. The click-fit top cover o-ring — A20212
The small o-ring on the top cover click-fit. Easy to overlook, and because it's small it's the one most likely to be missing entirely after a rebuild.
Replacement for Fluval A20212 Top Cover Click-Fit O-Ring (pack of 4)
Other Fluval models
- 105 / 205, 106 / 206, 107 / 207 — motor head gasket A20038
- 304 / 404 — motor seal ring A20063
- C2 / C3 / C4 — seal ring A20288
- AquaClear 20 & 30 powerhead — A16625
- AquaClear 110 & 500 — A16417
A note on lubricant and aquariums
Use a food-grade silicone lubricant on aquarium seals, and use it sparingly. Petroleum-based greases attack rubber and have no place near livestock. Aqua Flex is food grade and safe for this use. Wipe the groove clean first — on a canister filter, cleanliness matters more than lubrication.
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