When you say amphidinium cysts what do you see? A while back and still now actually I noticed kind of gold colored orbs that amphidinium were swimming around on my sand bed and some were actually attached to the individual grains of sand. I never got a clear answer as to what they were. I tried many things to have an affect on these cysts. The only thing that I clearly had make a difference on them was syphoning out the clusters, rinsing the sand and then completely drying out the sand preferably in the sun. When I dry them out under the microscope they look dried out and "dead" (who knows if they are actually dead). I have done this together with this method
https://www.reef2reef.com/threads/analyzing-a-bacterial-method-for-dinoflagellates-and-cyano.635165/.
This seemed to slow them down but has not completely gotten rid of the problem.