My common pleco - about 16 inches long, over 20 years old - is constipated. In hindsight, having read up on it, I suspect he got jammed up from eating tubifex worms. I put some in for the angelfish and he loved it. Came out of his cave to eat within seconds of my putting it in there. Now I have learned they are a real problem for plecos. Once it dawned on me what was happening I stopped all feeding except mashed peas, which he eats readily. I treated the tank with epsom salts. I did not want to try to put him in an isolation tank, so I treated the whole tank. I'd willingly lose the other fish to save him. It has been at least three weeks since he's pooped, and he is starting to look like a bowling pin. He is still quite active, comes out of his cave and searches for food, eats the peas quickly when I put them in there, perches on the rocks and looks at me (or seems to) - basically all pretty normal, except not pooping and swelling up. The treatments have not touched the issue. So at some point I guess I conclude either the tubifex worms formed an insurmountable blockage, or he has a growth or tumor blocking his gut and is not going to get better. When do I make that call, and what do I do to be humane?

Well, nobody commented so probably nobody will see this, but here's an update.
She (yes, she!) continued to swell up - eventually looked like she had swallowed a tennis ball - and I am not exaggerating. But seemed otherwise healthy throughout, so I stuck with her. About a month ago she started shrinking, and is now almost back to normal. So in fact it WAS eggs, which she eventually absorbed. Five month saga!

Wow, thanks for the update! I actually just saw this thread a week or so ago and wondered what the outcome was. I've heard of eggbound fish before but I hadn't heard of eggbound plecos!

In additional reading I learned that female plecos typically move out of the cave while the male readies it somehow - then she will go in and lay the eggs while he fertilizes them. Without a mate, they just hold on to them until they finally start reabsorbing them. I did notice she was staying outside her cave - lying on top of it more than usual. She was getting too big for it though, so I redesigned it, raising the roof another inch or so. She went right in after I did that, but continued staying outside a lot too. I guess my "readying" of the cave was not suitable for her!