My last Pleco is gone... what NOW?

[squirrelgoddess]

I detailed the problem and treatment of this pleco from my 10 gallon tank in another post:

Originally posted by squirrelgoddess
Update: We did indeed have Melafix at the store, and I brought it home and added it to my last surviving pleco's tank. The slight redness that I saw in him last night has progressed to cover much of his face, and I know now for sure that he's probably developing a bacterial issue of his own. What is plaguing these poor guys? Should I pick up some nitrate and nitrite tests from the store as well?

I'm going to be doing massive water changes and Melafixing in all of my tanks tomorrow... I'm praying that my bettas, danios, corys, swords, and snails are more immune to this horrible plague than my poor plecos have been. I'm hoping I caught this last one's infection early enough; from what I read in the disease database in the link Lotus posted, it looks like it's bacterial, whatever it is.

I'm frustrated because my fish are all in clean tanks with filters. The previous two were in tanks with Small World filters, which are admittedly not of the caliber of a Whisper (which I have on the 10 gallon), but the water in those other tanks is clear and there is little debris in the gravel thanks to the plecos and snails. Now this pleco with the red patches is from the 10 gallon, and that water is absolutely crystal with the Whisper filter on it. I understand that there are many other factors to healthy water and healthy fish, but I'm not familar with what they all are. What could the problem be?

So I just did a water change for this pleco because his algae wafers had clouded up the tank quite badly; evidently he wasn't interested in them. He had been attached to the sides of the tank rather than sitting on the bottom, which I wasn't sure to be a good or a bad thing. Anyhow, I put him in a cup, changed the water, added some Aquasafe and the recommended daily dose of Melafix, and put him back in. The water was a bit warmer than his had been, and I didn't float him, but I understand a slightly elevated temperature to be theraputic anyway. As soon as I put him in he dashed along the walls a bit, then shot to the surface and strained upwards like he needed air---then he popped down and just floated. It happened that fast. After a few seconds he started scooting around the tank again, but he wasn't moving very well. Then he floated again. That's where we are now.

I really thought I might have been able to rescue this guy, and the frequent water changes were recommended for that! What am I doing wrong? Was it the water temperature? I know fish will gasp for air and sometimes try to jump out if the temperature is too drastic for them; this water was well under 80 degrees, and he died pretty much instantly after I put him in.

I have a terrible feeling that it was the water temperature, though I've never experienecd that before. Maybe I wasn't as careful this time as I usually am with water changes; I always float them before transferring them back into new water, and I didn't do that this time. I wanted my pleco back in the hospital tank ASAP. I'm feeling incredibly guilty and stupid over the rash of fish deaths I've been having; there have been plenty of tears, but it doesn't seem like I'm learning enough because things keep happening. Please help if you can.