I don't know about the convicts, but a little bit of salt won't hurt your red bellies. I have 4 and added salt for a couple of crabs i added. the crabs nead it. I don't use the full dose. My directions say to use a rounded Tablespoon, but I flatten mine off. Better safe than soryy I guess. I have had no problems with it.
Some tips:
Mix it first than pour it in, I have been adding it to my filter after a water change. It desolves in the filter and get added more evenly.(just my opinion)
Don't pour on top of your fish, a concentrated solution could burn.
Keep in mind that salt does not evaporate or get filtered out. I only add more when I do a water change.
Good Luck!