Definitely Dino’s. I have turned off my skimmer, removed the majority of the chaeto, raised the sensors on my fleece roller, dosed phosphate, over fed, and turned on UV. Dinos still running rampant and phos still showing 0.
OK, so the ICP looks fine. You may want to add a second DI cartridge on your RO water.
I used to use a "silica buster" but found a second DI cartridge worked just as well and is a lot cheaper.
If UV is not knocking down the Dinos, I would take it off as it will just kill all the good things that you want to outcompete the Dinos. You most likely have a Dino strain that doesn’t hang out in the water column like Amphidinium so UV doesn’t help. Join Macs dinoflagellate support group on Facebook for more detailed information.
Do you have access to a microscope to confirm it’s Dinos and its type? A cheap one is good enough.
It’s possible it could be a diatom bloom, which is a much better situation.
If it's non UV treatable Dinos
First step is to keep dosing phosphate, put it on a doser, and dose 24/7. You want to be able to detect phos on a test anytime of day. I find phos gets bound up or used very quickly in new setups, so single large doses aren’t the best option. Going from high phos to near 0 will just stress your corals, fuel your dinos.
You need to get your biom healthy. I have tried all the different bottled bacteria and have found most of them to be useless (even if they say they kill dinos).
Stick to Prodibio Biodigest or real live rock/sand. Biodigest is pretty much the only bacteria product that members have tested that was actually a good marine bacteria mix and not just wastewater treatment bacteria.
Have people said they beat non UV treatable dinos with random bottled bacteria and phosphate dosing? Yes, have I, No.
I have beaten LCA and SCA a couple of times, and the only real fix was live rock and major phosphate dosing and monitoring. But the live rock will not work if your phosphate keeps dropping to near zero. It just gives the dinos the ability to outcompete everything good.
You may need to dose a ton of phosphate all day.
In this case, you’re going to want to make your own phosphate solution using TSP and a planted tank calculator.
It’s much cheaper and better than a lot of the products for sale that use potassium phosphate instead of trisodium phosphate.