I have a month-old aquarium set up, with 6 platys in it, a mixture of species. 5 of them are perfectly happy and healthy, but one is much slimmer than the others. I noticed it was ingesting flake food then spitting it straight back out, and now it will not touch food at all. It appears to be a weak swimmer. Does anyone know what could be wrong with it?

This post has been edited by Wippy M: May 11 2008, 04:33 PM

are there any signs of disease, a dulling of the skin or rapid gill movement, does it get bullied by the other fish? What are your water stats?

How many gallons or litres is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
How thin is the fish.
Check the anus to see if its enlarged or red and inflamed.
What does it look like when the fish goes to the toilet.
What do the gills look like, do they look pale with excess mucas, or red and inflamed.
Anys signs of flicking and rubbing against objects.
Does the fish look pale or darker in colour.

96 Litres (~20 Gallons)
6 small Platy (3 different species; two of each; don't know their names).
Don't know water stats, but that can't be the problem because all but one of the fish are perfectly healthy.
It's about half a centimetre thinner than the others, and a bit shorter lenthways as well.
The ribcage is a little more prominent than in the other member of the species, and the fins appear slightly shorter.
It does seem to flick and rub against things, not entirely sure if it's what you're thinking of though.
No change in colour.
It seems to get followed around by the other member of the species. The sick fish darts away from it then seemingly runs out of energy and rests on the bottom of the tank.

With the fish flicking and rubbing I would get your water checked.
Parasites can cause bacterial infections.
What do the fish gills look like.
If the fish is really skinny, it could be old age, internal parasites, to fish tb.
I would issolate the sick fish if you can.