Just so you know it could be phosphates leeching from the live rock fueling the byropsis.
Here is another mag post
I've battled for many months against this vile weed. Nothing I tried would eat it. I tried lettuce sea slugs (they eat it but not fast enough to control it), sea hares (aplysia sp.), almost every kind of snail commonly available in the hobby, 3 kinds of dwarf hermit crabs, emerald crabs, a small blue lined rabbitfish. Only the lettuce sea slugs ate it, but they only suck the chloroplast out and the plant still grows fine despite this. Plus they are easily killed by even the weakest of powerheads. I couldn't keep them alive in my main system due to the filtration and current.
Everyone kept saying there must be a nutrient problem or bryopsis wouldn't grow. This sounds good at first, but makes no sense when tests consistently show 0 nitratres/phosphates and you have a refugium with chaetomorpha growing. Cheato should eat up the nitrates/phosphates and keep your tank clean, right? My chaeto was getting choked out by bryopsis. Bryopsis needs extremely little light and nutrients to get by, even less than chaetomorpha needs. If a healthy tank (low nutrients) allegedly can not sustain bryopsis, then how is it logical to believe the same system can sustain chaetomorpha? Lowering nitrates/phosphates was not an option. I started using a phosban reactor, but how do you reduce phosphates beyond zero? This was a wasted investment, but at least it was cheap.
Then I stumbled on to this thread at Reef Frontiers. Mojoreef suggested boosting magnesium to 1500-1600. I don't understand the chemistry behind this, but for some reason the bryopsis absorbs the magnesium and it prevents it from growing. I didn't believe it at first, it was too easy. But it was cheap, so I gave it a shot. I ordered some Kent Tech M. I already had a magnesium test kit, and my mag always measured somewhat low (around 1100). I started boosting it up and within days the bryopsis turned translucent yellowish brown and started falling apart. I accidentally overshot my goal of 1550 magnesium and went to 1680ppm.
