• You are viewing the forum as a Guest, please login (you can use your Facebook, Twitter, Google or Microsoft account to login) or register using this link: Log in or Sign Up
  • You can now follow UKAPS on Instagram.

Leaking SunSun 304b, fixed


29 Oct 2021
Just wanted to relay the issues I had with a relatively cheap canister filter I got from the local fish group for free as the previous owner was complaining of it leaking.
I didnt see this fix anywhere so I thought I would post it for posterity for anyone facing an issue with this generic brand. These come in several different company labeling.
I got it for free so I did a water test, and sure as anything it had a steady drip coming from the upper assembly.

The main o-ring and cracked plastic near the hose connections were the only two fault points I could find online .First thing I checked the plastic connectors for cracks and found none.

For the o-ring, It had some sand in the area, and the retaining plastic was partially unglued. I carefully removed the retaining plastic, cleaned the glue off both it and mating portion of canister top with a super sharp wood chisel and reglued with gorilla gel.

When I put it all back together after also cleaning the oring and lubing it using some faucet silicone grease, it was still leaking. I tried the plumber tape trick, still leaking.

I was about to toss it, the night before trash pickup I hesitate and decided to completely disassemble to see if I could find the root cause or at least understand what makes this thing tick.

After popping off the white electronics distribution cover which only takes 6 screws, 2 from top, 4 from the bottom, I noticed the screw collar on the plunger assembly was loose, I still went about removing it all and greased up the gasket below it, and inside the plunger that moves when you push the button. When I put it all back together the leak was solved.

I'm positing that collar was loose enough from poor inplant assembly QC, possibly a lot of vibration or action on the priming button, it got to the point where the o-ring seal could raise above the collar or was not perfectly sealed due to the column being able to tilt slightly.

End summary got a working 300+gph filter for a gallon of gas, some faucet grease and some curiosity.
Last edited:


New Member
2 Feb 2021
Good job swyftfeet. I am wanting to toss my 2 sunsuns in favour of Oase filters, but for the 9 years I have had them, I have had minimal leaks, so cannot justify (to my partner that I need need filters :) )
In summary, here's what I have learned with using them.
1. Buy spare new o-rings. I got mine cheap from China and have some 9 spare at last count.
2. Use a pre-filter so I only open my filter approx 2-3 times each year.
3. I use the plumbers tape on the canister head and rest the o-ring on top of it using lubricant on the ring
4. Treat the filter gently, don't overfill it with media etc.
5. Pre-fill the canister with water before connecting with power.
They do the job on my 350 litre tank