Help Help!A friends livebearers are really sick!Ther stomach cavity just sinks in,and he needs help right now!!!!!

Okay this could be a few things, one under nutrition, step up the feeding regime!

Two an internal parasite thats sapping the fishes food before it can absorb it, treat with an internal parasite remedy.

Three, the fish are highly stressed and not feeding, test water parameters and tank mates for bullies/impurities or greedy fish taking the food from them.

Unless i get more information roothtooth, i can't give anymore information, try and get me a list of tank mates and water stats and i can be more help

Can you post a picture?

Another question would be how old are these fish? It could be old age, especially with guppies.