I've been battling bryopsis for several months now and nothing I do makes any difference at all to its growth. My nitrates and phosphates always measure 0ppm. I do use tap water, but my tap water always measures 0 nitrates/phosphates as well, as does my newly mixed saltwater. So I don't see the benefit of using RO/DI for battling algae. Lately I've drastically reduced feeding. I used to feed everyday, usually with frozen foods. Now I feed every other day, usually flake food. I've also been doing more water changes. Used to be once a month 25%, now it's once a week 35-40%. So far, no change in the bryopsis.
I added a phosban reactor several months ago, but knowing already that my phosphates were 0 I didn't think it would work. I was right, no effect.
I've tried several critters, none of which had any effect. I've tried dwarf sea hares (aplysia species, not dolabella which I hear are better, but impossible to find), lettuce sea slugs (they poke around on the bryopsis, but don't appear to actually eat it), several kinds of snails (cerith, nassarius, ilyanessa obsoleta, astrea, Mexican turbo, zebra turbo, Indonesian Turban, Queen Conch, red footed conch, nerite, trochus, margarita and I think one more kind). None of them touch it. Neither do my dwarf blue leg hermits or scarlet reef hermits.
The only other thing I can think of doing short of destroying the rock and all the corals living on it is to reduce the photoperiod. It's currently around 13 and a half hours per day (7:45am to 9:15pm).
The stuff even grows in my QT, which I never feed (no fish in it most of the time). Bubble algae also has no problem growing in the QT without nutrients, but it doesn't survive in my main tank thankfully.
