Thanks for the reply.. I'll fill in details, plus an update on my researching this problem.
1)What temperature?
My tank stays at 76.6, sometimes going up a degree due to sunlight hitting the tank in the evening. It is in the basement with west facing windows, so gets blasted for a bit. I'm going to install blinds at some point
2)What lighting are you using, and how deep is the tank? what is your photoperiod?
I'm using Radions Gen2/Pro. The tank is 22" deep I believe, lights on at 9AM and off at 6:30pm in an attempt to reduce the growth of this stuff, spectrum shifted toward blue
Sump is large (4ft x 2ft) a reverse photo period, filled with cheato
3)What are your water parameters, PH, Calcium, magnesium, and specific gravity?
pH ranges between 8.15-8.3. My pH probe has been giving me some odd readings as is my conductivity probe, but no reason to think things are off as my LPS is mostly looking fine (hammers, frogspawn, torch)
Calcium is 420-440
Mag is around 1400 (trying to rid myself of hair algae on a few rocks)
SG is 1.025 to 1.027
these are range values from over the past 2 months
I use an Apex controller to dose and control tank parameters
4)Are you using RO water?
I use a 3 stage RO. Fresh inserts and tests 1-3 ppm (tap water is near 200ppm)
5)How much circulation are you using?
The tank and sump volume add up to around 250-300g. I am using a Waveline DC 12000. Usually running full, but the power controller is sometimes really hot so I ramp it to 50% (need to solve that issue too)
I have a vortech MP40 on one side and a Jeabo WP40 on the opposite end of the 7ft tank.. seems like a lot of flow but also appear to be some dead spots, though I do have to have low flow areas for the LPS
(BTW.. I do NOT recommend the Jeabo pumps... 2 died on me and this last one will lock up needing me to spin the blade to get it started)
6)How is your skimmer operating? size, total system gallons?
I have an Aqua medic turboflotor 5000 shorty/compact protein skimmer..manual states its good for up to 400g. I have few fish in my tank. It pulls dark skim, but very little per day.
7)Could anything have gotten into the water by mistake, cleaners, kids playing near it etc?
No.
8)When was your last water change? what salt do you use?
I've had blooms of this single cell stuff for about 2 months, so frequent, large water changes. Last one was a week ago, roughly 50g. I've been using "Salinity" salt the past 6 months or so, before that Reef Crystals or Instant Ocean
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Over the past year I`ve dosed the tank with interceptor and treated corals with Bayer Complete to eradicate redbugs (which I now think I never had... learning curves are great fun) and AEFW, which I definitely had. The AEFW are gone.. Ive scoured over my SPS with a small handheld microscope and have seen neither eggs or the worms in over 2 months.
During that process, I lost my shrimps and all pods (which may have controlled the current issue I`m having possibly). Now that I have done many large water changes, I think I can re-introduce them.