You don't need to fiddle with this now, but if you get a new pleco type, try to quarantine it and maybe treat with a parasite guard. I don't think most of us realize how many of the conventional plecos (from fishfarm ponds in Florida) and most of the other suckermouths (usually from the wilds of South America and just starving by the time we get them) come in as tour boats for disease organisms on and in their bodies. They may be the vectors for stuff which proceeds to wipe out everything else in the aquarium.

If the fish in that aquarium of yours stabilize and thrive, you may just want to keep water changing. If you haven't been doing much gravel vaccuming, you want to do more. A lot of us (moi too) have been surprised by what is to be found there.

If that aquarium absolutely seems to be becoming "The Aquarium of No Return" you might consider boiling the gravel and bleaching the hard parts of the tank and equipment.

I'm using a courtesy computer at a motel at the moment. Everytime I try and pull up a second window to check Guppylog references, it deletes what I had on the first page. So I can't go and copy references. But if you want  a discussion on Bleaching tanks check with Immediate Help or click on my name, click on uncle's logs and then click on A Better Solution to Pollution or something like that.

Hope the water changes and light fish load in that troubled aquarium allow it to "right itself." Then none of that other heavy duty work  will be needed.