This article focuses on Cyano but the remedy is the same for Dinoflagellates too.
A scourge of this hobby is commonly known as "Red Slime Algae". It's NOT an algae but rather a bacteria - Cyanobacteria to be exact. Almost every hobbyist will deal with this issue at some point, most often at the end of a new tank cycling. The common name "Red Slime Algae" is deceptive as this bacteria can "look" brown, Purple, red, even black depending on the lighting above it & it's stage in growth/thickness. Most commonly seen in the beginning stages as a "reddish/brown dusting" on the sand/rocks if left untreated it will quickly become thicker, even stringy looking, can have little bubbles trapped in it & carpet your sand, rocks & even your corals! Cyanobacteria is super toxic! It can & will kill corals, fish & inverts! Pics below of how it can look - quite different yet all the same bad stuff! Cyano!
Cyanobacteria is a bacteria that is always present in our tanks. When it grows in enough numbers so it becomes "visible" & more "physical" it is because it has a food source allowing it to grow. That food source is primarily high levels of dissolved organic carbon compounds (DOCs). Cyano "blooms" or "breakouts" can be caused by just one ... or a combination of the following factors:
Poor skimming - dirty or plain ole ineffective units
Not using an RO water filter or you have an ineffective RO water filter, i.e., any brand not using a Filmtec membrane
RO membrane not fully "seated" into it's housing allowing the water to bypass the membrane
Infrequent water changes
Improper filtration, i.e., sand bed not deep enough &/or too little rock
Nutrients dumped from die-off on rocks during a cycle
Liquid invertebrate and gelatin based foods - HIGH sources of DOC
Old bulbs giving off a lower color temperature - Most bulbs need changing every 12 months
Poor water circulation, not enough water flow inside the tank & low oxygen saturation from your overflow system operating under the 10x per hour rate.
Nutrients unlocked from disturbing a sand bed in moving a tank or re-sanding.
How to get rid of it? First, correct any possible cause listed above. Then, physically remove the cyano (do not stir it up or try to net it out as this just breaks it up & further disperses it in your tank) by siphoning it out & turn off your lights for 3 days (your corals will be fine). Siphoning may require repeating daily if cyano spots reappear. Siphon until the cyano no longer appears. Bite the bullet & do it though! Siphoning & physically removing it is the "key" to success. Don't worry about sucking up a bit of your sand bed. It can easily be replaced (if necessary) with more live topper sand that doesn't need rinsing. Add ROX carbon & a or multiple Polyfilters to remove the toxins & DOCs already in the system.
This method WILL work...but only if you make it work...by doing the work.
BEWARE of chemical products that claim to rid the tank of this scourge!! I cannot stress this enough! The most common products are antibiotic based and will also kill your good bacteria as well as the bad thus reducing/eliminating your system's ability to break down wastes & increasing your DOC levels. Also, the toxins in the cyano are left behind & can continue killing too! Cyano WILL almost always return if only chemically treated and the source(s) of the problem (DOCs, water flow, lighting etc.) are not dealt with. You MUST be prepared to intervene as I stated above. It warrants repeating...correct the cause & siphon, siphon, siphon! Your tank & beloved livestock are depending on you.
I followed the advice on your website regarding the eradication of cyano and it seems to have worked exactly as you said it would. Yesterday was the first day back with lights on, and for the first time in 6 weeks I did not get any residual cyano growth the day after a water change! I've also switched up the eating regimen as per your advice, and although it's probably too early to tell how that impacts the health of my tank, I think it's safe to say that the cyano is gone for good this time!