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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).

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    Hi, could you please tell what exactly does the mud look like, maybe even include some photos? As far as I know, the brown or gray "gunk" or "grime" that accumulates in the sponge and parts with high water flow is actually the biomass of live nitrifying bacteria that facilitate the nitrogen cycle. On the other hand, this mud could also be composed of particles of decomposing matter; does the mud appear in any significant amounts anywhere else in the aquarium? What is the flow rate (in liters per hour) of your filter? Thanks. – lila Apr 24 at 14:26
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    The mud is brown only at those two locations. Topmost apoon (white) is completely covered as is the moving part of the filter-motor itself. Need to wait a few days before a photo is possible just cleared it yesterday. Mud itself is so thick that no flow rate is possible any longer within 3 weeks. Filter is a high rate filter for 180l (shop said its ok to have a bit stronger one). Would have to get to the shop to look at the rxqct value) – Thomas Apr 24 at 15:58
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    Maybe you can add the values of the last water test too? – Allerleirauh Apr 24 at 19:52
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    how often do you do partial waterchanges and what % of the water do you change each time,and how often do you vacuum the gravel/sand.how much food do you give and how often.please edit the information into your question. – trond hansen Apr 25 at 11:53
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    Answered for both questions – Thomas Apr 25 at 13:53
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It is fecal material and decomposing plant material. That is why replacement filter elements are available. I use polyester fiberfill padding sold to make quilts; I cut it in rectangles to fit my filters and replace it as needed. I also use the garden hose to flush permanent type filters. The mud material also collects in certain locations in my pond; I occasionally scoop it out and use it for garden soil enrichment.

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    Is it usual that one filter type doesnt really have that, but another has? (in the old aquarium...way less fish and 1/2 the size we never had any large mud concentratoins) – Thomas Apr 27 at 16:45

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