What is the best way to encourage coralline algae growth on live rock in a fish only tank? Specifically, what is the best lighting (intensity, spectrum, and photoperiod), additives, etc.?
Coralline algae is surprisingly easy to grow, and if taken good care of, can grow relatively fast in FOWLR tanks (fish only with live rock). How? Well, let’s look at the things that really effect coralline algae growth.
In terms of water chemistry, coralline requires only a few things different than your fish do. Like corals, they require calcium, magnesium, and some source of carbonate (alkalinity). So keep your parameters stable and in line using good calcium, alkalinity, and magnesium buffers. Keep your chemical parameters as follows:
Calcium: 380-450 ppm
Alkalinity: 2.8-4.3 meq/L, or 8-12 DKH
Magnesium: 1200-1400 ppm.
Also, keep your nutrient levels relatively low. High levels of phosphate are detrimental to coralline algae growth in two ways. First, there is anecdotal evidence that low levels of phosphate are directly inhibitory to coralline algae growth. Second, phosphate contributes to the growth of competing algae, such as hair algae. That algae competes for space, light, and nutrients with the coralline, and even grows directly on top of it. Other algae, such as diatom algae and cyanobacteria, also compete for space and light, so keep your silicates, nitrates and other problem nutrients low.
Surprisingly, many species of coralline algae actually prefer low to moderate light. In many very dimly lit tanks, coralline algae completely overtakes rocks, pumps, and even the back glass. If you have moderate to low lighting, moderate to low light coralline algae species will dominate your tank. So moderate power compact lighting, or VHO lighting should work great for you in a FOWLR tank.
If it’s still hard for you to grow algae, there are a couple tips and tricks that you can consider. First, try purchasing new liverock from an already established tank. Many fish stores will have older rocks that are completely covered in coralline algae, that they will not mind selling at a premium. Just buy a few pounds, since the algae will spread. Seeding your tank with a variety of coralline algae species will allow you to introduce a species of coralline to your tank that may be better suited for your lighting and water chemistry. Second, if you do have some coralline algae, try taking a new, washed, firm toothbrush, and gently scrape the algae until a light dusting of it come off. Supposedly, this encourages the spread and colonization of coralline algae spore. If your tank is currently running ozone or UV sterilization, turn it off for a while, as you may be hindering this spread and colonization of coralline algae spore.
If all else fails, there is anecdotal evidence that some amino acid supplements can help to encourage coralline algae growth. The effects are purely anecdotal, but the idea is promising.
Try any or all of the things above, but don’t expect to see results right away. Coralline algae grows relatively slow compared to nuisance algae. Expect to see results in 2-3 months, rather than 2-3 weeks.