We change or water every month. We have tried weekly, bi-weekly monthly. there is no difference in this growth. We use store bought RO water and do 10 gal water changes. The tank is 75gal with a 33gal sump. There are 3 clowns, 1 regal, 1 emperior, 1 damsel. We also have frog spone, zeina, star polyps and a couple of hard corals. We feed once a day with a 1/3 of frozen food qube. There is no macroalgae and we haven't tested for magnesium ever