A lot of people are providing various solutions but you have a much larger problem on your hands... that puffer needs to be housed in a minimum a 125 gallon tank. Even as a small juvenile as it is now, its far outgrown the biocube already. Doesnt matter what you try and do, but that much biomass, and not having any clean up crew, you're looking at a painful experience.
That all being said, doesnt even take into account the terrible home conditions for the puffer. In such a small tank you're going to be looking at stunted growth of the puffer, stress, and aggression when it cant find its own territory with other fish in that small space. Ultimately, that fish will suffer and die much earlier than its life expectancy.
I understand the need to buy fish and other critters that are a "must have" but you either need to provide adequate conditions of them, or limit your must haves to those that can live happily in what set up you have.
