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sorry if this is in the wrong spot, I'm a newbie to this forum!

I have a common pleco in my community tank that seems to be having some problems...

Basically, it's like he's too buoyant; when he sucks on the glass with his mouth, his tail-end starts to travel up the side of the glass until he's parallel with the ground, essentially. He ends up at the very surface of the water, with part of his body protruding from the water just a little bit. The poor guy tries really hard to "stay down" but it seems like he can't help but float up.

Remembering back to about a year ago, I'm pretty sure another pleco that we had (in this same tank) had the same problem, only we didn't really realize it at the time. He died.

What's the problem and is there a solution? anyone know!?

I posted a question very much like this several months ago when my common pleco was doing the exact same thing. Are you testing your water parameters and how big is the fish and the tank?
My pleco was getting too big for the 29 gallon tank I had it in and its problems were stemming from a decrease in water quality (I think particularly from pH drops which led to less oxygen in the tank and my pleco taking frequent trips to the surface to gulp air, leading to a severely bloated stomach). My pleco's symptoms were alleviated by a lot of water changes, nearly 50% every day, then close to that every three days thereafter. Its symptoms improved steadily after a few days of water changes and it was normal afterward as I continued the constant water changing. Recently I moved my plecos into a 55 gallon tank and the common pleco looks better than ever and hasn't displayed any of these problems with less frequent water changes (about 50% once a week). I have heard that the bloat can sometimes come from overeating, but this is less likely than it being from water parameter problems. If you find that pH is a problem due to lack of buffering capacity (alkalinity or KH), you will want to buffer your water more against pH swings.
But all of that said, START CHANGING A LOT OF WATER FAST. That was the gist of the best advice I got, and I really think it saved my common pleco (thanks Planet Catfish Loricariid forum!)

Thanks so much for the advice.....I will get on those water changes ASAP.

The pleco is about 5 or 6" and the tank is a 77 gallon. The other fish in the tank (one adult angel, 4 harlequin rasboras, 3 scissortail rasboras, 6 neon tetras, and a sailfin pleco) appear to be doing fine...but there is a lot of waste collecting (seemingly, from the sailfin pleco). I HAVE noticed the sick pleco going up to the surface for "gulps of air," like yours did.....so yeah....seems like the same thing. Thanks again, I'll keep ya updated on the progress (hopefully!).

Update:

Did massive water changes for days. Even pulled out all my dirty substrate (sand) without disturbing the tank.

Pleco is nearly dead though.....still extremely buoyant.

Hi. I would try adding some Epsom Salt (MgSO4) found in the drugstore used as a bath salt or a laxative. It will reduce swelling. You can dose at 1 tbsp. per 5-10 gallons. I'd dose for at the low range and observe for a couple hours and then increase it to the high range.
I haven't noticed MgSO4 stressing any fish I've treated, but I'd go slowly as you said your pleco is almost dead.
I'd also purchase some Metronidazole based medication and treat the tank with it, or if you don't want to use medication you can try using a herbal vermifuge like the several garlic products for aquarium use.