The calculator we used above is the website of the company/brand GFO I am using. Sounds like we're all in agreeance that the correct dosage would be around 2-3 tbspn for my 13.5gallon evo, so I will start with just 1 tbspn and see how that goes.I don't know the basis for the calculator, but I'd go by the manufacturer recommendation (typically something like 1 tablespoon per 4-8 gallons) as a starting point, or half or a quarter as much if you are very concerned about bottoming out, and then adjust by trail and error.
I typically used a half to 3/4 cup at a time in my reef tank (120 display, 300 gallons total) and changed it after a few weeks.
Thanks for the help everyone.


