Interestingly enough Red Slime Algae isn’t algae. It’s a type of bacteria called Cynobacteria. Characterized by a reddish or bluish snotty looking mat on substrate, Cynobacteria is the result of excess nutrients, organic material, lighting, nitrates and or a combination of the above.
The easiest way to get rid of Cynobacteria is to maintain proper water parameters along with routine maintenance.
Excess Phosphates as well as improper lighting are common culprits of Cynobacteria. Try to keep Phosphate levels below .03 ppm along with an adequate photo period relative to the type of tank.
Water Parameters: Most aquarists have a basic understanding of the foundational water parameters for beginner saltwater tanks;
(parameters below are for a 55 gallon mixed reef tank with LPS coral such as Frogspawn and Aussie Torch, a few mushroom coral, clown fish, Yellow Tang, Six Line Wrasse, Tail Spot Blenny, Skunk Cleaner Shrimp and assorted hermit crabs and snails.)
ammonia: 0 ppm
Nitrate: < .05 ppm
specific gravity: 1.025
A more common omission from standard parameter tests is phosphate. Excess phosphate equals excess algae and Cynobacteria. There are many reasons for high phosphate levels. Some of the more common culprits are water changes with non RO water along with certain sea salt mixes. Carbon and other filter media can also add phosphates to water although if the media is from a quality source this should be minimal.
Too much organic material such as fish waste and detritus will increase the nitrate level, which in turn will provide a source of nutrition for Cynobacteria. Siphoning debris from the substrate along with a NO3:PO4 reducer will help control nitrates.
I keep my lighting in between 70%-80% for my violet and blue’s while my UV, greens and reds hover around 10%. My Hydra 25 HD LED usually stays around 40% cool white as well. I tried adjusting my lighting parameters in an attempt to get rid of them, but ultimately it was the photo period that needed adjustment. The usual photo period was 10 hours and as soon as I corrected the water parameter levels and lowered the photo period to about 8 hours they went away completely.