Upside-down catfish have swollen bellies

[courtneylm]

So I got 7 upside-down catfish about a month ago, they were about an inch long when I got them and now they're 2-2.5 inches long. I've been noticing recently that several of them have pretty swollen bellies. I know it's hard to breed them in captivity so I don't know if it could be eggs, could it be that they're just eating a ton? They seem to be acting pretty normally, and water parameters are good: Ammonia 0, Nitrite 0, Nitrate 20, pH 6.6 (yes I know that's a bit low but I haven't had any luck bringing it up, and that's what my tap water is at). Has anyone who's kept these notice that they sometimes look this way? I don't think they're being overfed but maybe they are.

This one is the most swollen

This one isn't as bad

 

[Demeter]

Mine look about the same, fat females I think. Maybe you will see spawning activity seeing as you have a good group of them. Not sure what their breeding habits are like, you may want to look into that so you know what to look for.