The anti-phos sponges are designed specifically for small tank users i think. Smaller tanks usually have this problem of space constraint which will proberbly stop you from using proper FRs to run your medias like Rowa and carbon. The sponge combines both carbon and Rowa into 1 neat little sponge, hasn't used it personally before but heard some good feedbacks from users. Show a pic of the filter you are using and im sure we'll be able to find someplace for you to put the sponge.