Originally Posted by djembekah
I think OFL has successfully kept multiple males together in a tank, but that's only after many, many generations of breeding. you'd have to ask her about it.
Yeah, she does but she didn't use just random males from the store, which is where the OP is getting them, she has kept them together for at least a couple generations and were bred to be peaceful. Almost all bettas at the store have just been bred for cash.
Unless you divide the tank you can't keep male splendids together, they will seek each other out and fight, the weaker one might try to flea, but the stronger male will just seek out the other and kill him whether stress or literally biting him. A lot of wild males can be housed together though, just make sure they are in proper pairs.
How much current do you have? I know that tetras like more flow and for such a big tank you don't want a betta in there if it is going to be blown around, especially a long gunned males.