i agree with herefishy with pretty much everything. even if you dont want
to breed bettas you should get at least
one more female so that your male wont just chase and hurt the opne you have. i had a betta pair that did this and it wasnt fun.
i also think that your platies are most likely pregnant. im not sure if this is correct but i heard somewhere that guppies are the only ones where you can see the gravid spot. i had mollies that were pregnant and they didnt have the spot. so if you or anyone knows the answer i would like to know because i also have platies that i want to breed and i would like to know when they are pregnant.
thanks. and herefishy is definatly right about getting a bigger tank not only so that the fish have personal space but also becuase you will want many more fish once you get going. trust me. i started with a small tank and now have a 29 gal. tank. and i still want a bigger one! but i have to say, it is pretty impressive that your male betta and male platy dont fight. good for you!! :D