Six Different Algae and their uses
There are several different genus of algae available to the home aquarist, and more than a few that we no longer stock because of the risk of (or history of) becoming invasive. Most alga is easy to keep, desiring parameters well within the ability of every aquarist. Happy and dominant algae cultures will deplete alkalinity and to a lesser extent calcium. Regular water changes and or dosing nutrients in extreme cases can help keep them flourishing. Algae for the marine aquarium can be divided into a three camps, with some overlap. Some make for very underrated display pieces, others are primarily used for nutrient export, and the third class is primarily for food, there is obviously substantial overlap between these categories for certain genus of algae.
Nutrient Export Algae
The most commonly used algae for nutrient export are Chaetomorpha sp. and Ulva sp. Both of these benefit from 24 hour lighting and as such are primarily used in refugiums or otherwise dedicated chambers in sumps. Neither of these is a great display alga as they both tend to just float around. Ulva in particular is very likely to float right up to your return and immediately clog it. The plus side of Ulva is that many herbivores love it, so it can be harvested from your filtration system and clipped into your display where your tangs, rabbitfish and others will graze on it. Chaetomporpha has a texture that is unappealing to many of these same fish, though it does act as habitat for many micro-organisms which will give your system a small, but constant supply of copepods that flourish within the Chaeto.
Just place them somewhere in your filtration system, give them lots of light, and harvest a portion on a regular schedule. What you harvest can be given to friends, or your LFS, or tossed in the garbage. Be careful not to toss too much of it in your compost as it brings quite a bit of salt with it. Chaeto can deal with substantially more flow, and will grow in a nice neat ball if given the correct pattern.
Food Algae
The most commonly used live macro algae used for food in home marine aquariums come from the genera Gracilaria and Rhodophyta. Both of these can also be used for Nutrient export, though Rhodo grows somewhat slowly, and both of these have a tendency to fragment. This is fine in a feeding frenzy where all those bits are sure to get eaten, but in a tank with an inadequate number of grazing fish, all that material ends up in your particle filters. Both Gracilaria and Rhodophyta are nutritionally complex and are generally greedily consumed by most aquarium herbivores. They make a great complement to each other.
Both these algae are very easy to keep, having little in the way of flow requirements or light intensity needs. They do need some light, and some flow, but nominal amounts of each will keep them alive. If you desire to keep your Rhodo brilliant red and growing quickly enough to not have to continually buy it, more intense lighting and flow may be needed. Some people will keep a small extra tank with a hang on back filter, a small led light and a little powerhead just to grow the algae. This mechanism works well, but if you are keeping these for food, you may as well grow them in your system and get some benefit nutrient export benefit as well. Your plant eaters will take care of (and love) any break off that makes it into your display tank.
Display Algae
Both Gracilaria and Rhodophyta can be very attractive in a display tank that doesn't have any herbivores, though as noted, aquarists should pay extra attention to their mechanical filtration when doing so. There are two genera of algae that we sell expressly for display purposes and those are Penicillus sp (Shaving Brush) and Halimaeda sp. (Cactus or less frequently Coin Algae) are both slow growing algae and thus poor choices for nutrient export. Their beautiful forms and deep green color make them uniquely appropriate as display pieces. Both are unpalatable for most herbivores meaning they are pretty useless as food. Sadly, those same herbivores will frequently give these algae “tester bites” which will usually stress the algae to death over time.
As a result, for aquarists interested in growing either of these algae in their displays, we suggest utilizing them in aquariums without herbivores. Of all these algae sold, these two are probably the only ones that even moderately challenging to keep. They will need strong, non laminar flow, and moderate, full spectrum lighting; they can fail to thrive under lighting that is too dim, but are sensitive to burning by over bright lighting as well. If you have high intensity lighting in your aquarium, take steps to shade these additions and acclimate them to high par locations gradually. Shaving brush coral will also need enough substrate to stand up in and fine grain sand is preferred for this, whereas Halimeda will generally come attached to a small rock, or can be glued to a rock with superglue. In addition, both these algae have slightly more intense demands for free calcium and alkalinity in the water than most algae. If you are using a high quality salt like Tropic Marin, and doing weekly water changes, that will be enough for all but the most impressive stands of either of these algae.