Hello, just for the sake of anyone popping by this thread in the future, the conclusion I've gathered so far is that whatever my eels' situation is, it is not fatal. In fact, three months later, I'm thinking there isn't a problem at all. My eels are a bit heavy in the stomache area, or were, not so much, and they seem to be just fine with it. As for the veins/dark objects that were showing... I think that may just be part of their stomache. It makes sense that I wouldn't have noticed them before, the gravel they were on perviously was dark, and so were they. In a natural-gravel tank, they got lighter. Thus, some of their internals probably got easier to see.
Either way, it's been three months and I have a pair of eels that are growing, clever, healthy, and happily take part in the food scramble with a mob of bouncy weather loaches.
For now, case closed. Eels: Totally fine.
Cat
