Start here — before replacing any parts
Check the easiest restrictions in order: hoses, media, fittings, then the impeller.
This information comes from a shared HW-series reference; check the UV module, rotor and electrical rating fitted to your HW-304B.
Manual scope: Check canister/hoses and correct revision
Do these checks in order
Follow the steps below and keep a record of what changes after each one. Stop when an actual cause is confirmed.
Unplug the canister. Follow both tubes from the aquarium to the filter, removing twists or flattened sections; open any intended flow-control valves.
Inspect intake and outlet fittings for buildup. Remove and rinse obstructing debris according to the manual.
Open the canister only as directed and clean clogged mechanical filter media; handle biological media in removed tank water to avoid unnecessary disruption.
Inspect the impeller cavity and rotor for grit, wrapped fibers and broken components. Reassemble every seal and reprime using the prescribed HW-series priming method.
If the problem is gone
Normal flow returns after one restriction is removed: that restriction was likely responsible; monitor for leaks after reassembly.
If the problem is still there
Clean hoses/media and impeller still yield weak flow: inspect for air leaks at the fittings and check whether the impeller/shaft is worn before ordering verified parts.
Which cause matches your result?
Use these more specific checks only when the earlier steps point to that possibility.
01Impeller chamber is blocked
Inspect the impeller and its chamber for gravel, shells or other debris.
Clean the impeller and housing without damaging the components.
02Inlet or outlet fittings are clogged
Inspect the removed inlet and outlet adapters for buildup.
Clean the adapter interiors using suitable tools.
03Hose is kinked or compressed
Follow the full hose routing and check for bends.
Reposition the hose to restore a clear path.
04Inside of hose is obstructed
Inspect the tubing for accumulated debris.
Flush and clean the tubing as directed in the manual.
05Filter media is congested
Inspect the media for excessive buildup.
Rinse appropriate media in removed aquarium water rather than untreated tap water.
How can I tell it is actually fixed?
Compare return-jet strength at the same outlet position after priming and observe for leaks/noise over the next operating period.
Before buying a replacement part
Impeller and rotor shaft are revision-specific; do not match solely by appearance.
Where this guidance comes from
Primary reference: View the manufacturer or manual reference ↗
Applicability: Check canister/hoses and correct revision. Source evidence: Matching HW-304B owner case plus manual.
The instructions are original English troubleshooting explanations derived from the listed technical facts; they are not a verbatim service manual. Do not apply them to an unverified revision.