you have some really pretty fish there, but horseluver is right, you need a much bigger tank for the danio and tetra, and they are schooling fish, so at least 4 more of each would be nice. also, bettas are tropical fish, so unless you house stays around 75 to 80 degrees at all times, yes, you do need to get him a heater.
From Horseluver50 on 06/09/09
This is to small of a tank for all these fish, get a 10 gallon, put the tetra and danio in there, leave the betta in the 3 gallon...
From veganchick on 03/16/09
very pretty, but tetras and danios are schooling fish that need filters and heaters. bettas don't need a heater, but they do best with one
WhiskeyHands's 21 gallon freshwater fish tank, Betta Tank