Those are ostreopsis. They move around as if they have the pointy end "tethered", right? Proro are round. Ostreo are GENERALLY the easiest to solve for. A properly sized UV with SLOW flow will do wonders. They are also the most toxic of the bunch so running some GAC is advised.
I wrote this article back in early 2022. Sorry for the long read but it should help.
As the title suggests, this is intended as a short guide on what to do when you suspect dinoflagellates are trying to overtake your system. It is an attempt to boil down the protocols discussed across 11,000 posts in this "Are you Tired" thread...
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