I noticed patches of red velvety material on the substrate of my saltwater aquarium. What is it? It sounds like you have the beginnings of Red Slime Algae. If left untreated, these patches will grow into an unattractive velvet-like blanket.

Red Slime Algae are reddish-brown in color and first appear in your aquarium in small clumps or patches. If left untreated, the algae will spread and form a thick, slimy layer across your entire aquarium.

To prevent nuisance Red Slime Algae from overtaking your aquarium, routine maintenance is extremely important. Here are some simple steps you can take to reduce excess nutrients and organic debris, which play host to Red Slime Algae:

Use a siphon to clean a portion of your substrate regularly. This will remove organic debris and uneaten food. Also, be sure not to overfeed your fish. Only feed as much food as they can consume in a few minutes.

Be sure your aquarium has good water movement. A powerhead or wavemaker will help increase water movement.

Perform regular water changes and routinely replace mechanical filter media.

Maintain proper marine pH between 8.1 and 8.4 with buffers or with kalkwasser. A high pH helps discourage aggressive growth of Red Slime Algae.

Supplement existing filtration with phosphate-removing chemical filter media.

Use a reverse osmosis system for quality source water. This will help prevent buildup of excess minerals in aquarium water from the get-go.

Red Slime Algae are an annoying addition to your aquarium, and once present, a challenge to eliminate. The best way to deal with Red Slime Algae is prevention. With a regular maintenance regimen, you can help keep your aquarium beautiful and algae free.