Whenever. I've been sugar dosing in the 30 and 90 but the 30 is building a nasty slime in the sump and in my drain as return lines. I've replaced all fittings and plumbing and finally contacted Red Sea about their po4x nitrate and phos reducer and they said that's a downside to sugar dosing in small tanks. Tired of cleaning it out every couple weeks so going back to media.