I have had this prob before where the danio stops schooling and appears to be sulking, by just staying in one area of the tank, circling around, very unsociable. I had three danios, all leopard, but one died and behaved in this way too, but it never did school with the other two, and was always a loner. Despite being a leopard danio it was a different colour(more yellow) and was added/introduced at a later stage, so the other two appeared best friends and the newbie just remained alone. very sad. Any advice pls.

I have a Fluval Edge(23 litre/6 gal) tank
two danios
two balloon mollies
one guppy(the other 3 died)
one red comet platy

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Fluval Edge,(23 litres/6 gal) Weekly 25% water changes.One guppy, two balloon mollies, one platy and two leopard danios:

I'd suggest having a minimum of 5 and make sure your water is right.
Do you have a reliable test kit (not strips) to read your PH, ammonia, nitrItes and nitrAtes?

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150G, 1 shovelnose sturgeon 8", 1 polypterus endilicheri 9", 1 leopard ctenapoma 4", 1 Distichodus Sexfasciatus 6", 1 Farlowella acus algae eater 8", 2 silver dollars, 2 tank bred angelfish, 1 clown knife 11", 1 Jaguar hybrid syntadonis 4", 6 African cichlids 3+"

Thanks for your suggestion. The danio in question does seem to have recovered somewhat and is currently chasing around with the other one. These two have been together since I set the tank up. I know it is better to have at least 4 danios, however, my tank is only 6 gals and with the other fish in there I think I may be over stocking it. In addition,I have two balloon mollies, one platy and one remaining guppy.
I do have ammonia testing kit and go to the aquarists stockists regularly to test for other things and my water has always been fine. I get the feeling there could be some sort of bacteria in there as I have had quite a lot of sick fish, resulting in death.My plants have all gone brown too, so I fear something is definitely lacking!!!

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Fluval Edge,(23 litres/6 gal) Weekly 25% water changes.One guppy, two balloon mollies, one platy and two leopard danios: