I would be concerned over that. I have little to no experience with diseases so I will not guess on treatments. Hopefully another member who has such experience will see this thread and be able to offer some guidance.
Firstly,, I'm no expert on fish diseases so forgive me if that which I mention is of no help.
I have seen plecos become bloated when they ingest too much animal proteins such as that found in fish foods, and too little vegetable matter such as spirulina pellet's, or flake,zuchinni,spinach leaves, or perhaps my personal recommendation,, (New life spectrum pellet's for large fish)
Even when vegetable matter is provided, these fish will often go for the scraps of fish food if care is not taken to keep proportions equal.
Too much of wrong food can encourage bacterial pathogens in gut of these fish.
Is also possible that fish has scraped it's belly on sharp object(s) in the tank like rocks (especially Lava rock) or,, possibly suffered burn from heater which these fish often seem to do.
Should go without saying, that water parameter's must be zero for ammonia and nitrites, and these fish prefer oxygen rich water with moderate to strong current (think air stones ,powerheads).
Hope some of this helps identify or eliminate possible contributer's to what may be ailing the fish.
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