Try cleaning with vinegar. I used vinegar and hot water to clean a tank that housed hermit crabs lol. You won't have to stress about the minuscule amount of vinegar residue harming your betta either :)
Hot water and vinegar? I also have another tank mum brought home and I have no idea where it's been and what's been in it. I cleaned it up the other day but I wanna be sure there's nothing residing that would hurt the fish I'm planning to put in it.