Help! Sick tank.

[sdb]

I don't know what's going on in my 33gal, but something's wrong. Two days ago I noticed one of my serpaes looking a little ragged and pale. I put him in the hospital tank and noticed what looked like a red bruise or bleeding under the skin on one side. He got worse, so I treated with Maracyn-Two, and today he died, rather quickly, too. One minute he was swimming, the next time I turned around he was down on his side, gone. He was bloated, too, and his eyes seemed a little large, which is why I treated (I thought either popeye or septicemia).
Now I've noticed my leapord danio (same tank) has a large red mark under his skin on one side, like bleeding. He's also bloated and lethargic and definately not himself.

I'm wondering about septicemia, but I don't know. I read it on my meds pkg, but I don't want to play guessing games. I've checked the other fish and they all seem fine, other than another serpae that looks a little fat, but I don't see any marks.

Here's my info:

Tank- 33gal, planted, CO2
PH 7.2, Amm 0, nitrite 0, nitrate 20, temp 78F
water changes every week (25%), no additives except aquaplus conditioner

Fish-3 ottos, 4 peppered cories, 4 harlequin rasboras, 3 serpae tetras, 3 pearl 1 zebra and 1 leapord danios


Any advice would be greatly appreciated. The hospital tank is still set up but still has the maracyn-two in it (one dose only was added).

 

[sdb]

I forgot to mention that I've been fighting algae in the tank, the kind that's bright green and covers everything in sheets (blue green algae?). Anyway, I've been doing daily water changes. This shouldn't be causing the problem, but I thought I should mention it.

Also, the red patch on him is now looking like a wound, but it's hard to tell if it's actually bleeding or just "redder" in one spot.