Algae growth is indeed very hard on seafans and gorgonia, and if algae is accumulating on your gorgonians you need to tend to it as soon as possible. Algae growth can be carefully removed using a soft brush and one of the best ways to help prevent it from growing on the gorgonian is to increase the current or water flow over the gorgonian. Try positioning a small powerhead so that its output moves water over the branches of the gorgonian where the hair algae tends to grow. (If seahorses are present in your reef tank, be sure to shield or screen off the intake from the powerhead so that the seahorses are protected.)
Here are the directions liveaquaria.com provides for carrying for gorgonians when algae becomes a problem:
"In order to inhibit algae and cyanobacteria growth, it is important that this gorgonian is provided with a medium to strong, constant or intermittent water flow. If in the event that it does begin to become covered with algae or cyanobacteria, remove it from the coral immediately, as this will cause rapid tissue deterioration. Algae can be removed gently with the use of a soft brush, and cyanobacteria can be combated by soaking the coral in freshwater of the same temperature for approximately 1 minute."
Corals in general, including seafans and gorgonia, do not tolerate full strength hyposalinity at a specific gravity of 1.010 very well, so when reefkeepers treat their tanks with hyposalinity they do not lower the salinity as much as you would an efficient only tank. Reefkeepers and hobbyists with sensitive animals usually do a modified version of hyposalinity where they lower the salinity to 1.017 rather than 1.010 (Giwojna, Dec. 2003). The delicate animals generally tolerate 1.017 well and although that’s not as effective in eradicating parasites, a specific gravity of 1.017 is still low enough to provide many of the benefits of hyposalinity (Giwojna, Dec. 2003). So your nursery tank is at a specific gravity (1.016) that is close to the hyposalinity a 1.017 that most corals can tolerate fairly well, Seagazer.
Most corals are safe at even lower salinities, but 1.017 usually provides adequate protection and provides a margin for error. In any case, as a rule, reef keepers DO NOT take their systems lower than 1.015 for safety’s sake (Thiel, 2003). (This is also a good level of hyposalinity for hobbyists who have only a typical pet-store hydrometer for measuring specific gravity, or anyone with many invertebrates in their seahorse setup.)
Corals typically close slightly immediately after the salinity is lowered, but are open fully again by the next day, and suffer no harmful long-term effects from hyposalinity at 1.017 whatsoever (Thiel, 2003). Reefers who practice hyposalinity report that it has no long-term detrimental effects on the growth rate of their corals.
According to Thiel, corals that are know to be sensitive to hyposalinity, and which are thus not well suited for OST, include Seriotopora hystrix, Montipora digitata, Pocillopora species and other similar hard corals with a fine, dense, polyp structure (Thiel, 2003). Acropora species, however, handle hyposalinity well and soft corals are also generally fine, including such sensitive softies as Xenia, Lemnalia, and the like (Thiel, 2003). As long as the pH and alkalinity are maintained at normal levels, most hard corals are not harmed at a specific gravity as low as 1.017.
I can’t say for sure whether your particular gorgonia would also do well at that salinity, but I suspect if you acclimate them to the lower salinity very gradually (drip acclimation would work best) when you transfer them from your reef tank to the nursery tank that they may be okay. At least, most corals can tolerate a specific gravity of 1.017… But if you want to give it a try, I can tell you that the polyps on your gorgonia should not be harmful to the seahorse fry.
You might also contact with the guys at reefcentral.com — they are the experts on reef tanks, and some of their members may have better suggestions for you on dealing with the algae on your gorgonians. They can probably tell you from firsthand experience whether your gorgonians would tolerate the salinity in your nursery tank.
Once your seahorse fry have made it past their pelagic phase, and have settled down to a bottom-dwelling existence where they orient to the substrate and seek out hitching posts, it’s safe to very gradually return the nursery tank to normal salinity (1.022-1.025). To avoid potential problems with the height reach, it’s important to adjust the salinity upward again slowly. Returning the salinity to normal over a period of days is safest.
So if your fry are hitching regularly now, Seagazer, you can begin gradually adjusting the salinity back to normal. Once that is accomplished, you could try rehabbing your gorgonians in the nursery tank without any concerns about the salinity or specific gravity.
I would say that you are doing just fine when it comes to rearing, Seagazer! All seahorse fry are challenging to raise, and there is always a steep learning curve when you first give it a try, and if your survivorship is increasing from brood to brood, that’s a sure sign that you are on the right track.
Best of luck with your rearing projects, Seagazer! Here’s hoping your gorgonia recover quickly.