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I do not like to use GFO in a tiny tank. If you really want to use it, start with a tiny amount. Like 1/4 of the recommended amount. Test phosphate often. It may tend to decline quickly, then rebound when you remove GFO as phosphate is released from the rock and sand calcium carbonate surfaces. It can work, but use a tiny amount and monitor. When it stops declining, add another tiny amount leaving the old stuff in.

