you can fit another in there, but it may be a bit cramped. You don't need an air pump, but having one is always better. As for sand, almost anything will do, but like tlindsey said if you want the best bang for your buck pool filter sand is the way to go. If you wanted other sand you can also go with garnet or black.Definitely sand, any specific type? Also what can I pair with it and should I get an air pump? I was hoping to just have 2 fish live in there, but I don't want any fighting or anything obviously.