Red algae

Even after we've eliminated "blue-green algae" or cyanobacteria, the "brown algae" or diatoms, and the photosynthesizing single-celled euglenoid protists that cause "green water," there are two kinds of true algae you may find in the aquarium: red algae (Rhodophytes) and green algae (Chlorophytes). The two aren't related at all.
 
Red algae. Black brush or "beard" algae, both in the Audouinella complex and other species, are red algae, or Rhodophytes. All but a couple of dozen are marine forms. Though as a group the rhodophytes are a minor presence in somewhat acidic and somewhat alkaline fresh waters from Brazil to Finland, Australia  to Hawaii, black brush algae is the frustrating bane of some well-managed long-running aquaria. Sometimes in a LFS I'll see plants and gravel, filter stems and rocks sporting dense growths of black brush algae, which doesn't seem to trouble the fish store staff but makes me think, "Can I really afford to buy fish from this tank?" 
 
Black Brush Algae at first will form a dark tuft of fine threadlike filaments arising from a single point, often on the edge of a leaf. Eventually it will spread to form patches. It's slow to get started, but so tenacious it can't be removed from its favored sites along the edges of broad slow-growing leaves (Anubias is a favorite) without damaging the leaf. Bit by bit it can swamp a tank, as at the local fish store. It's so difficult to eliminate that it rates its own "Brush Algae" thread among algae notes at The Krib.  That's where you'll find Eric Olsen's photo illustrating black brush algae, if you're in any doubt about whether you've got it or not. And be sure to look at Neil Frank's rogue's gallery of red algae portraits at Aquatic-Gardeners. The best single long article on red algae and how to avoid it and deal with it  is by Neil Frank, once again. Frank, the editor of The Aquatic Gardener who first popularized Siamese Algae-Eaters, summarized several methods for "Control of red algae in the freshwater aquarium" in a 1996 article in The Aquatic Gardener and in another version published in F.A.M.A., November and December 1996, now archived for you at The Krib. What he has to say about red algae — "beard" or "brush" algae —  often applies to other kinds of algae. In spite of his confusing lapse calling these algae "Rhytophyta," if you follow up only one algal link here, this is the one.
 
But George Reclos successfully treated black brush algae with a syringe of hydrogen peroxide, though at some cost to his plants. Read his detailed and excellently illustrated article, "Fighting the algae with hydrogen peroxide", archived at the Malawi Cichlid homepage. 
 
Phycoerythrin. The Rhodophytes do have chloroplasts that produce the familiar green photosynthesizing pigment chlorophyll. It's the same chlorophyll a originally invented by cyanobacteria that other algae use, but the green is masked by an auxiliary photosynthesizing  pigment, phycoerythrin, that is unique to the red algae. Phycoerythrin and other phycobilin pigments absorb blue light and reflect red, which gives many marine red algae a pronounced bronzy-red cast.
 
Almost all the red algae have stayed in the sea, where they include reef-building "coralline" algae and the red nori that wrap sushi. In the small group of freshwater red algae, which include Audouinella, phycoerythrin levels are reduced. So black brush algae is not always very black, sometimes more of a purplish-gray.
 
Few fish are willing to eat "red algae:" Otocinclus maneuver round the dark tufts as they work over a leaf surface, leaving them standing up like thistles in a cropped pasture ...but Siamese Algae-Eaters, Crossocheilus siamensis, are generally undeterred. You can bet that their track record on this score is closely followed: some aquarists are convinced that SAEs only crop the new growth and don't eradicate the basal growth; this was confirmed for me recently when I removed two half-grown Crossocheilus that had eliminated black brush algae in a 10-gallon tank that was planted entirely with the slow-growing gracile form of Microsorum pteropus. In a matter of  ten days or so, the scourge returned at full strength. I broke down the tank and disinfected it with bleach, binned the old gravel and Flourite substrate mixture and put the plants in a dark closet for three weeks, in a tank with a splash of water in the bottom, closely covered for humidity. The plants have come through but are still under closely watched isolation. The solution is to introduce the Siamese Algae-Eaters before black brush algae overwhelms the tank.
 
I'm inclined to prune out any leaf showing some Audouinella tufts along its edges. It's no problem! even slow-growing Anubias is more inclined to replace a leaf if you'll first remove one. Gravel showing tufts of black brush algae should be siphoned out and discarded. Sometimes I drag the edge of a credit card over the gravel to roll algal growth away from the light. A mulch of beech leaves discourages algae on the substrate in some of my tanks. Other ornaments can be boiled, bleached, rinsed and sun-dried... and the stuff might still come back: Lazarus algae.
 
It's been noticed that red algae are especially troubling in aerated soft waters with plentiful free CO₂, like mine. I can attest to that. Neil Frank noted that when a LFS buffered a display tank to neutral pH, red algae died back considerably, and he notes that they're never a problem in an alkaline Rift Lake aquarium. Brush algae are troublesome when aquarium waters are high in nitrate and phosphate, and free iron in the water encourages problem algae too: keeping your iron fertilizer in the substrate, not in the water, is part of the answer.