I have 3 male guppies with yellow tails, I have had them for 2 months and all of a sudden one of them has declined. Its tail that is always fanned out is pulled right in and its top fin is flat to its body it also isolated itself from all the fish in the tank.
For the moment I have isolated him in a breeding net feeding him little and often although he is eating he is leaving food.
I have very recently introduced 5 danios and 4 platys into the tank which the pet shop I use assure me will be OK together, I have always had guppies mollies and neon tetras in the tank together and have always got along well.
I need to get to the root of this problem if the guppy gets better its pointless me putting it back in with the rest for it to happen again. Any help will be greatly appreciated.

This is due to an illness, possibly brought in by the newcomers,overcrowding or poor water parameters.
What size tank and filter do you have, and how often and how much are you feeding? What is the ph and ammonia levels?
Remember stress will bring on illness. Due to too many fish and or over feeding, which will both bring on poor water.

Look out for ich. First sign treat it quickly and it won't be a big deal.