My best guess is that this is a bacterial infection.  Have you tried an antibiotic treatment?  I like Pimafix myself, but your local pet store can probably help as well.

It's an internal infection so you'd have to treat this with injections of antibiotics. Although, if their bellies are red already, I'm afaid there isn't much you could do...

I'm sorry to hear that we couldn't save your fish. Truly I am.. I hope all goes well in your future fish keeping..

Im sorry! 

I think the plants carried the disease velvet in to your aquarium which quickly transferred to all your fish.  The disease multiplies quickly in very clear warm water with strong lighting.  On white colored fish it looks like rusty red ich.  Healthy fish are lost in only 2 or 3 days.  I have had success with removing fish to a bucket with just an air stone and feeding the fish well while letting the water go very murky in low light conditions.  The velvet grows more slowly and spreads a lot less as the spores find it harder to attach on to the fish when the water is all murky.  Velvet is a tricky bug as part of its cycle involves photosynthesis.  So while everyone is aware about intro of bug carrying fish the plants seem harmless.