Fertilizing macroalgae

[Nevin]

Hi,

I'm looking for some advice on growing macroalgae faster. I have a small refugium in the sump with plenty of 6700k light on a reverse lighting cycle(picture below). When I transferred to the current tank I used RO water and I have only one fish and don't feed much, so not a lot of nutrients in the water. The chaeto had already shrunken to a tight little ball in the old tank and hasn't grown since. The red macro grows, but has been decimated by my urchin.

Anything I can do to increase growth? I dose a SMALL amount (as I can't test levels) of Flourish Nitrogen. Is growth typically limited by nitrates? CO2? Other? Do I just need to get more fish to fertilize it?



Thanks for any help,
Nevin