guppie in trouble

[Lawplanet]

i woke up this morning and the guppie is just laying on the rocks.. i don't know why. his one eye seems to be really really black.

all the other guppies are ok.
i don't know about the water parameters but i change the water everyweek. bout(15-20%)
i also have tetras angels and paties in the tank and they are also ok. actually not really. 1 pattie is on the ground as well.
it's a 20 gallon tank, and i decloriene the water before adding it to teh tank.

p.s. i'm a newbie to fish keeping. didn't realize it was so much work

thanks

 

[William]

Sounds serious but I don't know what it could be so I'm "bumping" this post to the top for someone else! Hang in there!!

 

[William]

Just thought of something If you've got any other fish in the tank this won't be a good idea but if its just livebearers you could try buying some aquarium salt as this works wonders on many fish problems, if you do have other fish in there you could try a salt bath for the particular guppy