Macroalgae WONT grow!!??!?!!?

[Mako]

I have a 15g and a 20g tank connected (reef system). I'm trying to grow macroalgae and it just won't. For the first 6 mos, it grew like an oscar on steroids, now it just sits there all green and pretty, same size as it was last week.

Nitrates are 15ppm, not terrible but not good either. I want to get this down but the macroalgae isn't helping (avoiding a dsb at the moment).

I DO dose with chelated iron. No response whether I dose heavy, light, or not at all. It does perk up with a 30% water change, but only for a couple of days. What nutrient could this be?

[OrionGirl]

Phosphates? Calcium? At a guess, calcium is the more likely of the 2. Also, trimming regularly typically promoted growth in my FO when I had lots of macro in there.

[mogurnda]

What kind of macro? Ulva seems to stop when phosphate and nitrate get too low. I can't remember chaeto ever slowing down. I've always avoided caulerpa, so I guess I have no opinion there.

How's your alkalinity? From what I understand, the major carbon source for marine macroalgae is carbonate, so they may slow way down if alkalinity is low.