I am so sorry to hear your babies aren't doing well Creat! Little red dots on their bellies before they die....hmmm. Do you think it could be a internal bacterial infection? I'm just pondering on that thought because of the red bellies and the clamped fins with no visible external symptoms. If so, isn't metronidazole good for mixing with food for treating for anaerobic bacterial infections, it also clears quickly with carbon filters and has no impact on the bacterial bed.
I don't know if this helps any but I hope the rest of your babies pull through!