Then really listen to me.
Then you shouldn't have adopted him. Plus, it isn't 30-50. Its 55-75. That is absolutely not an excuse. Look on Craigslist. They give away large tanks all the time. Or rehome him.
You don't, do you? Then there is no hope. They can get 2 feet long. That wouldn't even FIT in 2.5 gallons! Secondly, they produce so much ammonia and poop, they need double filtration even if they were in 75 gallons! You need to change the water 2ce a day 100%!
Then wow. You should give him his whole life. That is awful.
*2 Bettas (for now RIP Ryu) *2 Goldfish *9 Guppies *1 Gourami *2 Chinchillas *2 Budgies *2 Dwarf Frogs *1 Dog *1 Cat *2 Hermit Crabs *2 3gs *1 10g *1 Pond *2 2gs