Gunk

mhancock · Jun 13, 2026

Hi,

I rescaped this tank a while ago, initially it looked great but over time lots of fine brown light stuff accumulates on the white sand. The fish have also moved around the blue gravel that was meant to stay on the left!

I think it's algae from the pipes as after a water change when I turn the external filter back on a load of gunk comes out of the outlet. The pic shows what I caught in a little net from the outlet after turning the pump on. I can remove some by vacuuming, or by waving a net over the sand which lifts it from the surface, but of course that never gets all of it.

I also wondered if it could be from where the little plec has been gnawing at the wood.

Any advice? I was considering running the outlet through an old, not powered, internal filter in order to catch gunk in the foam which I could rinse off during water changes. I realise that this would reduce flow rate.

Thank you

GaryE · Jun 13, 2026

mhancock said:
Hi,

I think it's algae from the pipes as after a water change when I turn the external filter back on a load of gunk comes out of the outlet.
I also wondered if it could be from where the little plec has been gnawing at the wood.

It might be a misread on my part, but does this mean the external filter is off at times, beyond when you maintain it? To keep it cycled, it should run constantly, 24/7.

I see debris there, and not an algae. It has to be mechanically removed - siphoned out as part of the water change routine, or removed from the filter as part of the regular maintenance. Any mechanical filter (designed to removed matter from the water column) will cough up gunk when it's turned off and then on. If it's a quickly clogging filter, remove it and rinse it out more regularly.

It does look like your catfish is rasping off wood. I had Panaque that made piles of dark sawdust in the corners of their tank - piles I removed with my weekly partial water changes. They were the fish equivalent of the hair my dog sheds.

mhancock · Jun 13, 2026

Thanks for the reply, appreciated. Yes, the filter is always on except for water changes.

Out of interest, how often to ppl clean out external filters?

GaryE · Jun 13, 2026

I open mine up, and rinse them in tank water. There is worry chlorine in tap affects the cycle, but there's data showing it doesn't do much harm.

If it's a canister filter, you have to be ready with a long list of swear words to get through the process. Some models are a fight to restart, but restart they do.
A Hang on the Back, HOB filter is easy. Remove the sponge and rinse it. Rinse out the filter chamber. Refill and fire it back up.

If you have slot type media, there are easy fixes to eliminate the need to rebuy media. Not changing the filter inserts can give you far better filtration though - for the manufacturer, it's bad for business. Depending on the model, you can use wide pored sponge inside and get superior biological activity not destroyed with media changes. What model and make do you have? Brands aren't all international, but most follow the same principles.