I tried almost everything, from increasing temperatures to 28 °C to UV, increased nutrients etc. The only thing that killed Amphidinium in my case was silicate dosing, and a final blackout once they were almost gone. 3 months of battle solved in 3 weeks once I started dosing silicate. Since then I kept dosing Si in low amounts (0.2 ppm/day). I suspect that what you observed is not the direct effect of temperature but the increase in micro-organisms abundance caused by higher temperatures and bacteria/Si dosing.