Quick answer

If your SUNSUN canister filter is leaking from the seam between the canister body and the motor head, the O ring is the most likely cause. Order the correct genuine replacement seal for your model, remove the old ring, clean the groove, lubricate the new ring with silicone grease, and reseat it evenly. If the leak continues after a correct replacement, stop and inspect the canister body and hose connections instead of buying another seal.

Confirm the leak is actually the O ring

Before ordering parts, dry the outside of the canister with a paper towel and run the filter for 10 minutes. A leak that reappears along the horizontal seam between the body and the motor head points to the O ring. Water running down from the hose connectors, the primer button, or the intake and output fittings is a different problem and a new O ring will not fix it. Also check whether the leak only appears after you open the filter for cleaning; a ring that was pinched or twisted during reassembly often leaks immediately, while an aged ring tends to seep slowly over days.

Order the right seal for your model

SUNSUN HW series filters use model-specific O rings, so match the seal to the exact model number printed on your filter. The HW-304A and HW-304B share one size, while the HW-602A and HW-602B use a smaller ring. If your filter is a similar but different model, email the seller with your model number and measure the old ring before buying, because a ring that is slightly too large or too small will not seal. Avoid unbranded seals sold as "compatible"; a poorly molded ring can deform under pressure and cause the same leak you are trying to fix.

Replace the O ring without creating a new leak

Unplug the filter and close the shut-off valves, then remove the motor head and lift out the old ring with a plastic card or your fingers, never a screwdriver that can scratch the sealing surface. Wipe the groove and the mating surface with a damp cloth and dry them completely; grit or dried biofilm left in the groove is a common reason a new ring still leaks. Apply a thin film of silicone grease made for aquarium use, seat the new ring evenly in the groove with no twists, and reassemble the head straight down rather than at an angle. Fill the canister, open the valves, and watch the seam for 15 minutes before trusting it.

When to stop replacing and look elsewhere

If a genuine replacement ring still leaks after a careful install, the problem is no longer the seal. Inspect the canister body and motor head for hairline cracks, warping from heat, or a damaged sealing lip, and check that the head latches are pulling down evenly. A cracked or warped housing cannot be fixed with a new O ring, and continuing to tighten or add grease can make the leak worse. At that point, replace the filter or move to a different filtration style rather than buying a third seal.

Leak diagnosis by symptom

  • Water seeping along the canister seam: The O ring is dried, flattened, or pinched. Replace it with the correct model-specific seal, clean the groove, and lubricate the new ring lightly. If the seam stays dry for 24 hours after replacement, the repair is done.
  • Leak appears only after cleaning: The ring was likely twisted or seated unevenly during reassembly. Open the filter, reseat the existing ring flat in the groove, and check that the head sits level before latching. If the ring is still soft and uncracked, you may not need a new one yet.
  • Water still leaking after a new genuine O ring: Inspect the canister body, motor head, and latches for cracks or warping. A damaged housing will not seal no matter how many rings you try. Stop buying seals and either replace the filter or switch to a sponge filter for low-flow tanks.

Parts that match this repair

  • SUNSUN Canister Filter HW-304A/304B Replacement Rubber Seal O Ring: Genuine replacement rubber seal for SunSun HW series external filters, sized for the HW-304A and HW-304B. This is the correct seal if your leaking filter is an HW-304A or HW-304B, and it is the first part to try when water appears at the canister seam.
  • SUNSUN Canister Filter HW-602A/602B Replacement Rubber Seal O Ring: Genuine replacement rubber seal for SunSun HW series external filters, sized for the HW-602A and HW-602B. The HW-602A and HW-602B use a smaller ring than the 304 series, so this is the one to order if that is your model.
  • Extra-Large Double Bio Sponge Filter: Extra-large double sponge filter rated for aquariums up to 70 gallons, with a low-flow design suited to larger fish such as juvenile Arowana. If your canister housing is cracked or warped and cannot be sealed, this is a practical alternative filtration option for low-flow setups instead of repairing a failing canister.