Nuisance Algae:  Red/Brown/Black - Cyanobacteria

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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!

1. Overfeeding

2. Poor skimming - dirty or plain ole ineffective units

3. Not using an RO water filter or you have an ineffective RO water filter, i.e., any brand not a Kent

4. RO membrane not fully "seated" into it's housing allowing the water to bypass the membrane

5. Infrequent water changes

6. Improper filtration, i.e., sand bed not deep enough &/or too little rock

7. Nutrients dumped from die-off on rocks during a cycle

8. Liquid invertebrate and gelatin based foods - HIGH sources of DOC

9. Old bulbs giving off a lower color temperature - Most bulbs need changing every 12 months

10. Poor water circulation, not enough water flow inside the tank & low oxygen saturation from your overflow system operating under the 10x per hour rate.

11. 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 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.  At the same time you're siphoning, run phosphate remover & reef carbon media in a high flow area to remove the 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.