I never heard of raising mag having any effect on dinos. Raising the pH will help.
Problem Dinoflagellates and pH by Randy Holmes-Farley - Reefkeeping.com
"Raising the pH appears to help in some cases of problem dinoflagellates. In some cases when the pH is raised quite a bit (e.g., 8.6-8.8 or higher), the effect can be dramatic and rapid (within a few days), but if the pH is later reduced to normal, the dinoflagellates can return."
"Here's a series of actions besides raising pH that may help aquarists to deal with problem dinoflagellates.
1. Reduce available nutrients in the water. These include nitrate and especially phosphate. In a severe case, the concerns with driving phosphate too low may be minor compared to the dinoflagellates (and their toxins). In addition to the usual ways of reducing nutrients (skimming, growing macroalgae, deep sand beds, etc.), aquarists should consider very aggressive use of granular ferric oxide (GFO
). Putting a larger than normally recommended amount into a canister filter or reactor, and changing it every few days, may help. Don't bother to measure the phosphate level, because the goal is to have it well below normally detectable levels (say, 0.02 ppm
2. Reduce the photoperiod to four hours per day. This may help to keep the dinoflagellates under control, but by itself will not usually eradicate them.
3. Use more than normal amounts of activated carbon, and possibly ozone, to deal with toxins that the dinoflagellates may be releasing. This may allow snails and other organisms to survive while the dinoflagellates are still at nuisance levels.
4. Manually siphon out as much of the mass of dinoflagellates as possible. Daily removal would be preferable to keep populations at a reduced level."