Help my sick guppy

Hi all
Im looking for some help with my guppy.
I have a 60L Biorb with some neon tetras, 3 Black phantom tetras,3 Platys and i had 2 male guppys, I have had this set up for some time and all was well until a few weeks ago 1 of my guppys died, I checked the water and all seemed fine but now my other guppy looks very ill for the past week he has just been lying on the rocks at the bottom of the tank hardly moving i cant see anything obvious on him but now he is lying on his side and the only thing moving are his gills. I dont want him to suffer so can anyone offer any advice or would i be better putting him out of his misery.
Thanks
Gordon

 

If it's been awhile since your last water change, I'd do that. Changing about a third of the water every 2-3 weeks can do your fish a lot of good.

 

What are your readings for ammonia, nitrIte, and nitrAte (actual numbers, please)?
How often and how large are your water changes?

You're a bit overstocked (not horribly, though) so I'm guessing it has to do with water quality.

-Chelle