5 galling is better ... plus all tanks should be treated and cycled first. And guppies need heated water to give birth
Over all they need real warm water set at 80 degrees and use a water thermometer to keep track of it too
They all have off days, your parameters are good and you said he’s still eating and started getting more active. I wouldn’t worry too much unless he acts like it repeatedly and long term
Oh no I worry my head off! I have bettas and leopard geckos and if anyone of them acts slightly weird or...
Yewssss it's the right one... However you really need a small qt tank. You really don't need to treat the others if they aren't showing symptoms.... I understand you don't have the space. But he needs to be separated from the other fish asap.
Epsom salts are at any store in the pharmacy section. I also agree with gypsy on what all was stated. However if the fish is pine conning and losing scales from it it maybe too late. However to treat the sick fish he needs to be in his own qt tank