Ok- so a bit of background, I guess- my roommate has had this betta for at least a year, maybe longer (I've lived with her almost a year). And the poor thing was in a pint glass. More recently it was barely getting fed and was pretty much emaciated (not dead only because the other roommie and I were pitying the poor thing and feeding it/changing the water- roommate #1 has "just a fish" syndrome

).
Well, anyway- I got sick of that, got an old gecko 10g tank with mesh top (cat barrier) out of storage a couple months ago to at least give him some more room. Recently I learned about the heating thing- and I have a filter, too- but confused about how much turbulence is ok for them? The guy at the store told me I had to let all that settle before adding anyone else- I was maybe going to get one of those little cory cats or something to eat the food that's fallen to the ground. Food on the ground, being of course, inedible so far as the betta is concerned (maybe try flakes?)... and that brings me to my dilemma.
Since he's been gaining weight back I noticed recently that his front looks bloated- which I read on here means he's constipated. So I tried the pea thing and it sunk like a rock and I have a feeling he'll never eat it. So is there any way to make it float? Should I just wait and see for a bit? How long can I let the pea sit at the bottom before it gets gross and I should take it out? Is constipation deadly or should I not worry about it if I can't get him to eat it?
Thanks in advance and sorry about my mammal-centric roommate. I'm working on her.