Looks like you didn't research the mbu puffer. This congo species prefers temps from 75F-80F The water should be on the alkaline side with a pH of 7.5 to 8.0 (in my experience). Whereas your betta prefers a lower pH of 6.8 to 7.2. Didn't state what size the mbu is. If it's a recent purchase, these imports are prone towards picking up intestinal parasites. If it's eating, offer it fresh/thawed frozon foods with added garlic extract. The extract will purge the mbus gut of any hitchhikers. Foods can be thawed frozen shell-on shrimp, FW mussels/clams, SW mussels/clams, crayfish, snails, scuds, ghost shrimp, etc.
Be aware, the mbu grows to become a huge puffer requiring hundreds of gallons per puffer. This species reaches 30" in the wild and easily hits 2 feet in aquaria. They're messy eaters and they produce alot of waste. So, a highly efficient bio-filtration system is a must along with a regular tank maintenance program.
20 gallons is really too small for this species for the reasons stated above. A newly arriving mbu not moving is a major problem. It's probably stressed with pH shock if the pH is different than the lfs holding tank. And, the temp being that of your room can be a cause for temperature shock. If your room is mid to low 70s and the lfs tanks are around 78, that's too much of a change for this mbu to handle.