I emailed Seachem today to see how much SeaGel I should put in my Phosban 150 reactor and they replied very quickly, which was great. In case anyone is ordering some this may help you know how much to get: Thank you for your question, and also for your purchase of SeaGel! We recommend using 500mL of SeaGel per each 80-120 gallons of water. Therefore, since you have 55 gallons of water, you should use roughly 350-400mL of SeaGel in your reactor. We hope that you will be completely satisfied with SeaGel, and please let us know if you have further questions.