well for one, they do feed them, just not enough, most of their fish die in the tanks, but they arent so good at taking them out...the water DOES go stagnate. no filter can change that. wal-marts is never changed...
as for cheap fish tanks, just a glass globe. thats about the cheapest, and then you could save for a bigger one. i recomend a 10-15 gallon tank for one normal sized fish...
also dont put Beta fish with ANY others. they will either kill the other fish, or die trying.