I rarely post but wanted to chime in and report my success in battling amphidinium or at least bringing them back into balance. I had significant dino matting/streaming across my sandbed (~75%), and it was also covering 25% of my live rock. Yes, I did confirm with a scope and monitored it throughout my treatment. It was around 2 weeks before I noticed a significant reduction in the tank. At the 3-week mark, I could confirm under scope that diatoms were outcompeting dinos.
I run a 58gal mixed reef with LPS/SPS that has been established for over 10 years. Unfortunately, I was forcing too low (undetectable) nutrients with ZeoVit -- N and P tested at 0 for at least a year. I did dose various aminos and Sponge Power, but SPS color wasn't the best. My intervention and treatment involved: 1) adding a 36w UV running 24/7 via a cannister filter with 5 micron floss changed on regular intervals [note: UV is utilized on output], 2) removing Zeolite and stopped dosing ALL Zeo additives, 3) siphoned just the top layer of sandbed into a strainer over my sump using a 5 micron filter pad once a week, 4) turkey basting the rockwork each night right before lights out, 5) dosing Brightwell to slowly increase N & P to 2.5 and .01 respectively, 4) dosing Vibrant at 50% but 3x/week over 2 weeks, and 5) slowly increasing feeding of both frozen food and pellets. Luckily, this combination for me was a huge success, and at the 1-month mark I have no visible patches. TBH I haven't seen my rockwork and gravel look so clean and healthy in years. The coralline has colored up again and even shows growth. In retrospect, I hadn't seen any bristleworms, pods, spaghetti worms or even mysis dancing around in the caves for quite awhile. It does seem plausible that the microfauna slowly starved over time, and the opportunistic dinos took over.
Again, this is the outcome under my tank conditions, but I wanted to share the success using multiple approaches.