If it is dinoflagellate, reduce light in the tank to about 5 hours and keep up larger than usual water changes and replace carbon about once ever week. A clean and lower lit tank will stop the growth and the water changes will flush out the toxins that it produces. don't bother with snails and crabs. They will usually all die in a pile at the bottom of the tank within a day of being introduced.
You can try the raised pH trick using Kalkwasser, but the lower light regime is the way I have handled it in the past.