Hi Tami, 50 gallons is a really big fountain. I'd say 2 or 3 tablets might be more appropriate. For best results, it is best to (1) scrub the fountain clean (2) add fresh water, and (3) add the tablets in to prevent algae growth. If you have additional questions our Customer Service Department would be happy to assist you and can be reached at cs@smartlivinghg.com. Thank you, SLHG Team