SICK Discus!! Help!!

One of my 3 discus has been staying in one corner, has dark colors, won't eat and has been leaning on occasion. The other two are fine. Any suggestions?

Ammonia- 0
Nitrite- 0
Nitrate-18
pH- 6.5
Temp- 80

This has been going on for several days. I had one fish die about a day before and then had a nitrate spike to around 30 ppm. I've been doing daily water changes to get levels back down.
ALSO: I noticed what looked like slime coming off him when he has looked at his worst.

the other two def did not pair. the sick one is the dominant one and has laid eggs with one of the two healthy ones before.

Is it safe to do like 2 40% water changes in a day? Every time I do one he seems to brighten up.

Thanks, i'm starting now. I do think that nitrates have something to do with it. I've been keeping it at 10 but like I said, a fish died overnight (stupid platy's!) and I didn't see if for a little. I think after the ammonia from the corpse got cycled my nitrates went up up and away =p

No new fish. Just 2 platy's now. Although he is still in his corner he looks better again. I don't think i'll be happy until he eats again tho. He won't even take brine shrimp which used to be his fav snack. What about the garlic supplements? Do u think this would help his appetite?