Would you recommend to dump the one day old GFO and start over with fresh GFO, or should I add more to it?Sounds like 6 tablespoons is not enough. The recommendation is based on the amount of PO4 in the water. In your situation you are probably dealing with PO4 desorbing from the sand and rocks which can be a large reserve. Increase the amount of phosphate and just keep going. Measuring the reactor output is a good way to tell when the GFO is exhausted.