Dinos bloom under conditions of long photo periods of 12+ hours per day under more intense lighting like 20K LEDs for example. Additionally bringing phosphates to 0.00 ppm is a very good way to get them to bloom. Nitrates may not necessarily be 0 ppm but if they are it will encourage dinos to bloom more.
Dino blooms have been triggered when people switch to a new higher PAR lighting system, like 20K LEDs, and/or running too much phosphate removing medium like GFO or Phosban. I got them when I did both of these things.
While H202 can get rid of dinos, this course of treatment often results in them coming back due to the underlying condition of nitrates and phosphates still being too low with a long and intense photo period. This is also a hard treatment to put your corals and invertebrates through from what I have read.
Here is an alternate method that I used and recommend to get rid of dinos and prevent them from reblooming. You'll need to get your phosphate to at least 0.1 ppm and nitrate to at least 1 ppm. If you currently have a bloom, then also reduce your photo period and light intensity if you can. Manually remove larger clumps weekly to keep them from getting out of hand. It usually takes 6 months to fully recover from a bloom so you'll want jump on it quick. Running carbon for the toxins that some strains release will help prevent your snails and corals from suffering any health issues.
Measuring phosphate at these small levels is very demanding for test kits. The only two I found that are worth while are the Hanna LR phosphate or ULR phosphorus. While other test kits can have the refinement needed for these trace levels, the color scales used to determine the value are a pain to judge at best and many people find they don't match anything close enough to even make a call. But you definitely need a low range tester because something like an API tester just doesn't have the refinement.
