Fish stock for a Lake Malawi aquarium / Slimy layers

There are 7 Pseudotropheus zebra, 3 Aulonocara jacobfreibergi, 4 juvenile Labidochromis sp. "Yellow" and 4 Sciaenochromis ahli in my Lake Malawi tank, furthermore 8 loaches and 1 pair Bushy Mouth Ancistrus.
the loaches and the catfish do not harmonize well with Lake Malawi cichlids, and therefore should rather be kept separately instead. Also, it is not recommended to keep Mbuna cichlids (in this case: Pseudotropheus, Labidochromis) and Utaka cichlids (in this case: AulonocaraSciaenochromis) together since their nutritional requirements differ strongly.

The tank has 200 liters and an external filter. My Anubias have been dying for quite some time now. When I cleaned the filter yesterday, everything was covered with a film or some mucus. I then changed about 100 liters of the water. What has happened there?

Slimy layers are usually due to uncontrolled multiplication of bacteria or ascomycetes, however the latter rarely occurs in running aquariums and can  typically be found on roots in newly set up aquariums.

The cause for this multiplication of bacteria (that is very likely in this specific case) is probably that the fish stock is too high considering the tank size. In other words: The aquarium is simply overburdened biologically, even if some of the fish are not yet fully grown. Considering the Lake Malawi cichlids, you should calculate at least 300 liters each for the Mbuna and the Utaka.
Can the bottom gravel be the cause?

I do not think this is very likely, although different grain sizes are not favorable. This leads to smaller grains sticking in the gaps between the larger ones, thus clogging the bottom gravel. In the worst case, low oxygen (anoxic) zones  may form within the bottom gravel, where toxic substances (e.g. hydrogen sulfide) can develop.
What kind of bottom gravel would you recommend?

Round grained quartz sand or fine gravel (even grain size) is preferable. Be sure to avoid sharp edged material.

Best regards
sera GmbH

Dr. Bodo Schnell


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