Hi there Orestes428. FBTs usually need a tank that is half land and half water (you can accomplish this by sloping gravel to one side of an aquarium and using a screen to place moss or cocoanut husk on. A good filter for fire belly toads would be a tetrafauna filter for reptiles and amphibs. With your plan of 3 to 4 frogs, a 10 gallon can hold 3 frogs so a 15 to 20 gallon should be adequate for your frogs.