I've had a 120 L aquarium for a few years; about 40 small fish are in it (including 6 Corydoras paleatus and 12 fire tetras).
Now, as of a few months ago, I've noticed that the filter, the sponge, and even the mechanical parts are time and again covered in mud, and the effectiveness of the filter (thus how much water it pushes around) evaporates quickly when this happens.
This leads to both having to be cleaned every 2-3 weeks tops. After putting some bacteria in (a hint from a local fish shop), it became better for a bit, but now the problem is there again.
The water parameter values seem OK when I check them (according to test results at least).
So my question is, what can I do to get that thing under control? Try again with bacteria? Or is something else better?
Edit 2021-04-25: Checked water values:
- NO3- is too high;
- KH value is also not good;
- pH value I think is okay, but the colors from that test are quite near the border between okay and not okay).
Feeding is once per day and enough that the food is gone in minutes.
I do a partial change of water every two months, about 25%.
I do a full change with cleaning the soil every 6 months. The last one of these was two months ago.
Currently the filter works for 1-4 weeks until it needs a change due to mud (worsening every 3 days).