Unlike most algae, purple coralline algae are usually welcome in marine aquariums. They grow slowly, but have beautiful red to purple coloration. They get their name from the fact that these algae incorporate calcium into their structure, like corals do.
Most stores that carry saltwater fish and invertebrates do not specifically sell purple coralline algae. Instead, you purchase live rock. Live rocks are dead coral skeletons that have been colonized by other organisms. These often include purple coralline algae. In order to get purple coralline algae, find a piece of live rock in the dealer's tanks that has patches of purple, hard algae growing on it.
Purple coralline algae thrive under standard reef aquarium water parameters: a pH between 8.2 and 8.4, a specific gravity ranging from 1.022 and 1.025, and a temperature between 74 and 78 degrees Fahrenheit. Also, you need to keep an eye on your nitrogen compounds. Specifically, ammonia and nitrite need to stay at undetectable levels. Nitrate should stay under 40 parts per million, though lower is always better. Since coralline algae absorb calcium, and often shares a tank with other organisms that do as well, you should monitor calcium levels monthly. Ideally, calcium should stay between 380 and 450 milligrams per liter.
Purple coralline algae photosynthesize, so they require powerful aquarium lighting. The general guideline is to supply 5 watts of lighting power per gallon of aquarium water. Deeper tanks may need more light than this, since light attenuates as it moves through water. To supply this light, stick to high-output and very-high-output -- also called HO and VHO -- types of aquarium lighting. This includes T5 fluorescent tubes, metal halide bulbs and high-end LED arrays.
If their requirements are met, purple coralline algae require very little care. They generally grows slowly, and keeps other, less desirable encrusting organisms from getting a foothold in the aquarium. However, they will occasionally grow into aquarium equipment like filters and drains. Once inside, colonies can clog and block filters, damaging them. To prevent this, scrape the algae away with an algae scraper or razor blade if they start to encroach on aquarium equipment. It may take some effort, since these algae have a lot of calcium in their cell walls to protect them.