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Red Slime Algae AKA Cyanobacteria

The common name for cyanobacteria is "red slime" algae. Red slime algae is actually a bacteria and not a marine algae. Cyanobacteria means blue-green algae and comes from its color yet there are many other colors of cyanobacteria. The one color of cyanobacteria that aquarist are most concerned with is red. Red slime algae can start out in small patches and then combine to cover a large area.

Red slime algae needs light and nutrients to grow.


Having aquarium lights with a kelvin rating below 10,000k can contribute to the growth of red slime algae in the aquarium. Usually this is not a problem with aquarist that run their lights in the blue spectrum. The par rating of your aquarium lights is also factor especially with the new marine aquarium LED lights being so strong. One more factor to look at that can contribute to the growth of red slime algae is the operating time of the aquarium lights. If the aquarium is being lit 10 to 12 hours a day, you may want to consider turning them back to 8 hours until the red slime algae is under control..


The Dissolved Organic Compounds in the marine aquarium is the biggest contributing factor to red slime algae. Phosphates (PO 4) and nitrates (NO 3 ) are the two thing that need to be most looked at in removing red slime algae. Take the phosphates and nitrates out of the marine aquarium and you will starve the cyanobacteria out!

Phosphates (PO 4) are commonly introduced into aquariums by different means and the following should be looked at as a way to reduce DOC's. Using unfiltered fresh tap water, inadequate R/O system, or filters in the R/O being worn out should be one of the first places to look at for phosphates. A TDS meter (Total Dissolved Solids) should be used to check your R/O system for any problems and problems found should be corrected before continuing.

Many aquarium products may contain some phosphate but a big source for phosphates in the aquarium is the foods for feeding the marine fish. Cutting back on your feeding if you can may help reduce these phosphates. If you can't a good clean-up crew to eat the excess food will help. If you're not feeding too much to the fish and your water source is good, you may want to reduce the bio-load in the aquarium or look at other means of removing phosphates. You can use a phosphate reactor to lower phosphates but don't forget about the nitrates. You will need to lower your nitrates to be completely successful in getting rid of red slime algae.

Bio Beads can be used with great success in helping with nitrates and that alone may be all you need to beat red slime algae. Rarely used this days but also work well wit nitrates is a sulfur denitrator.

Nitrates are the final byproduct produced in the nitrogen cycling process. Allowing excess DOCs to accumulate in an aquarium can raise nitrates (NO 3) to a high level. Water changes and removing detritus can help with bringing nitrates down. If that does not work you may want to look at replacing the substrate. If the substrate is old and deep it can hold a lot of nutrients that the red slime algae will feed on. Having a good current flow in the aquarium will help prevent detritus from building up on substrate in low current flow areas in the future. Good current flow can also help the good bacteria in the substrate to break down nitrates better.

Anything rotting or dying in the aquarium will also contribute to DOC's and should be removed quickly. Uncured live rock and food will fall in the category and with the food, having a clean-up crew like crabs and snails to remove the un-eaten food will help with water quality. Some snails will even eat red slime algae!

Additional equipment and products to be considered for reducing DOC's:

Protein skimmer


Sulfur De-Nitrator

Sump and additional Live rock

Phosphate media and reactor

Bio Beads

Removing red slime algae from the aquarium will not be a quick process and may take a month or two to turn around. Don't be tempted to use some of the red slime algae removing products. These products cure the problem quickly but are only temporary because they do not fix the problem. Some products can also be harmful to the reef inhabitants and the much needed good bacteria.