Biomedia filters will have a gravel like substance in it. They will come in different varieties, but mostly should be like little holey hydro balls almost. Might be worth ebaying/gumtree/preloved for a glass tank, over time to upgrade him. Cheap plastic tanks just aren't big enough for commet goldfish once they get going. And I have never owned one that "only grows to the size of the tank". I have a 12 year old fish I won at a school fete and he is 7 inches easy, and still growing. But they do like having fish friends!