Have the same issue. Interestingly enough, with my old tank, they grew like weeds. Now, I cannot keep them alive. The biggest difference between the old tank and the new tank is the fact that I use Prodibio (Biodigest, Bioptim and reefbooster) to keep my phosphate and nitrate undetectable. My old tank used to have nitrates around 15-20. Never even really tested my PO4 but did about 30% water changes weekly.