Hey, I would say he should be able to live out his life in the 300gal. The Tetra Pond Filters look like pretty good filters but I have had no experience with them. Also, I have heard some good things about the Bioforce Pond Filters . If you go for a filter like that you will still have to buy the pump for it. For 300gal, I would get something that pumps at least 350 an hour (more IS better). I would get a MagDrive pump. They can be used in or out of the water. I would put the pump outside of the tank and just use hoses and put a strainer at the end. That way it won't get clogged as often. Hope this was of some help... Chris D.