Typically it's based on water volume, but I attempted to treat two very small gigantea on separate occasions and both died. I personally think the standard 250mg/10 gallons is too strong for such a small anemone (3"). The reason I believe this is because both were surviving without antibiotics -- one for 5 days with deflation, but reinflation atypical of an extremely sick nem, and the other for over 3 months but never with full inflation (also atypical of a sick nem) and I tried Cipro to see if I could simply "kick" the infection. Both died on the first day or treatment, and both inverted completely. Again, all anecdotal, but I think if you're trying to treat an extremely small anemone, it's something to consider. In the future I will use 125mg for such small nems and see how it works.