Someone said african clawed frogs...you CANNOT put a clawed frog with a betta..ever..they get huge and will tear your betta up. If you had a larger tank you could try african dwarf frogs. Honestly I don't believe a 2.5 gal is humane to keep any fish in. I think any fish would be bored in a tank that small. Maybe you'd find him more active if he had more room to swim around and explore. Laying on the bottom? What's your water temp? Bettas are most compfortable at 80F. Also is this a brand new betta?? Maybe the water current from the filter is a little strong for him, or might take some getting use to. You might want to put a tall plant, one that goes all the way to the top of the water, right under the filter current to help deflect it and see if that helps. One of my male bettas laid around for the first 1-2 weeks I had him and then he perked up and started being more active. His conditions were perfect so it seems this was just his way of settling in.