Hi guys. Would you mind helping me figure out what to do with one of my otos? He is clearly not doing well.

First, tank and maint info:
Tank size: 55g
Occupants: 4 boseman rainbows, 8 black skirt tetras, 2 guppies, 4 otos (down from seven as of Sept. 2012)
Temp: 78F steady
Params: 0,0, approx 20
WCs: >50% every 5 days
Light: High
Planted: Yes, moderately
Otos have cucumber to eat most of the time. They love to eat cuke, but do not eat much algae. 3 of 4 otos look OK, all other tankmates OK.

Second, symptoms: I have noticed that his tail has been rough looking for a couple of weeks, watched it carefully. He's skinny but still eating. Today I see him with many white spots on his lower body and his tail is eroded to almost nothing.

So, what could this be, and what should I do? I have very little experience with illnesses (except gill flukes and wow that's bad). Is it ich? I have read that I should up the temp of the tank to 83F or so. Should I do that in quarantine? I was wondering if quarantine would be a good idea since it would stress him out so much. The last oto I quarantined was very stressed by the experience.

Thanks for your help. I'm sorry if I sound stupid, I probably am stupid about this sort of thing. I've been lucky for the most part, and so I have no clue how to treat this.