First off, I apologize if my terminology is off. Please ask for clarification if you need it to understand something. English is not my native language, and I virtually never read about aquarium stuff in English, so I had to translate many aquarium/fish-specific terms!
I'm helping take care of a 160 liter (42 gallon) freshwater aquarium, housing a variety of different life.
It has about two dozen of a guppy variant (Endler's livebearer, Poecilia wingei), a few Lyretail killifish (Aphyosemion australe, 1 male, 3 female), one clown killifish, three L catfishes (one L128 and two sold as L201 that likely aren't), some shrimp (white pearl and 2 Amano), and a few snail variants, most of them tiny (about 1/4 inch).
The catfishes were purchased a few weeks back, along with some sinking pellets for them to eat.
Though I'm not certain of the connection, it seems that some pellets went uneaten, and since then (OR possibly since before), there's some black stuff in the bottom gravel. It's a fair amount, though; I doubt that all of it is simply uneaten food! My GF, who has the aquarium, is well aware of overfeeding issues, and AFAIK hasn't used the pellets very much since she noticed they weren't eaten.
In the past week, especially the past few days, most of the life in the aquarium has started to get sick.
One australe male plus a clown killifish died last week, and we found a dead shrimp yesterday, along with a sick one.
The dead ones last week were found the day after a water change and cleaning of the internal filter, where some (or a lot) of bad stuff from the filter came out into the water during the filter removal.
Today, plenty of the snails were headed for above the water level, and an australe female is lying on the bottom gravel, alive but clearly not well. The fully grown australe female looks fine to me, and the third one I'm unsure about, but she seems to have some energy, at least.
Another water change was performed, but with no apparent effect on the dying australe female. The others seem better to me, though.
Test strips seems to indicate that the water quality is good -- according to the parameters measured, of course.
The most recent one, from this morning, shows:
(General hardness) 4-7 degrees
(carbonates) 6 degrees
I hope that's enough info. I'm sure more can be provided if necessary.
I feel that in order to get rid of the black stuff, the aquarium might have to be fully emptied of plants, fish and decorations, and that's no easy task. Plus, there's only a 86 L aquarium (half of the current one) available to put them in temporarily.
Does anyone have any advice on what to do to take care of this?