A filter that stops working is one of the most urgent problems in fishkeeping. Without filtration, ammonia builds up within hours in a stocked tank. This guide helps you diagnose what's wrong quickly and get your filter back running.
Step 1: Check the Obvious Things First
- Is it plugged in? Is the socket working? (Test with another device)
- Has a surge protector or GFCI socket tripped?
- Is the water level too low? Internal filters need to be fully submerged
- Is the filter intake blocked by gravel, a plant, or decoration?
Step 2: Diagnose the Type of Problem
Filter running but no flow / very low flow
This is almost always a blockage. Start by rinsing the filter sponges in old tank water (never tap water). Then check the impeller, the small spinning component that drives water. Hair, plant roots, and debris wrap around impellers and stop them turning.
To clean the impeller: remove the filter head, pull out the impeller, clean the impeller shaft and housing with a small brush, reassemble. Most filter problems are solved by cleaning the impeller.
Grinding or rattling noise
Grinding almost always indicates a dirty or damaged impeller. Clean it as above. If noise persists, the impeller may be chipped; replacement impellers are available for most models and much cheaper than a new filter.
Humming but not moving water
The impeller is likely seized. Disconnect the filter, remove the impeller, and check if it spins freely by hand. If not, clean thoroughly. If the impeller spins freely but still no flow, there may be an airlock.
External canister, no flow after maintenance
Canister filters need to be primed after maintenance. Most have a priming button: press it several times until resistance increases, then power on. If no priming button, fill the intake pipe with water using a turkey baster before connecting.
Clearing an Airlock
- Internal filters: tip the filter on its side while running to let trapped air escape
- External canisters: disconnect the outlet pipe briefly while the pump is running
- HOB filters: pour a cup of tank water directly into the filter body while it's running
When to Clean vs. Replace Filter Media
Filter sponges and bio-media should be cleaned gently in old tank water, never replaced entirely unless physically falling apart. The media contains your beneficial bacteria colony. Replacing it means restarting your nitrogen cycle from scratch.
💡 Never clean all your filter media at once. If you have multiple stages, clean one stage per month in rotation. This preserves bacteria while maintaining flow.
Emergency: Filter Off for More Than 2 Hours
Your bacteria colony begins dying after 2–4 hours without oxygenated water flow. If your filter has been off for an extended period:
- Get it running again as quickly as possible
- Test ammonia every few hours over the next 24 hours
- Do a 30% water change if ammonia rises above 0.5ppm
- Reduce feeding to zero for 24–48 hours
- Add bottled beneficial bacteria to help re-establish the colony faster
When to Replace Your Filter
Replace when: the impeller or housing is cracked, the motor has burnt out, or replacement parts cost more than 60% of a new unit. When buying a replacement, always choose one rated for at least double your tank volume.
Find replacement impellers, filter sponges, bio-media, and replacement filters in our Filtration range.
