Have been fighting an issue for months and am hoping you can help. The problem is having red algae on the aquarium substrate/gravel. The pattern always seems to be the same - I do my regular water change on Saturday (10 percent only RO water), including vacuuming the gravel and it looks great. Then by Tuesday spots start to appear and by Friday it's back in all its glory. The alga goes away when the lights are off and returns during the day. I have tried almost everything I can think of except tear down the tank. All of the livestock is doing really well so I certainly don't want to do that. I have a 35 Gallon Red Sea and all of the seawater levels appear to be perfect, nitrate and phosphate zero, calcium 425, alkalinity 3.5 meq/l, pH 8.2 and temperature 80. I regular dose calcium, baking soda, Iodine, and a little magnesium. I have tried changing all of the filters, changing the lighting, add Phosban (normally I use Phosguard), reducing the time lights are on and added a wave powerhead to adjust the flow. The bio load is low, 1 clownfish and 1 coral beauty. I also use Tropic Pro as my salt mix. Some very small amount of red algae accumulates on the rock, but it's very small and is a fraction of what is on the gravel. The only thing I have not tried is removing the gravel and replacing it with fresh substrate. Any ideas on anything else I could try. As I mentioned the livestock is all very healthy and looking good so I don't want to rock the boat with any chemicals. Thanks for your help and I look forward to your reply.
Thanks for your letter and there are two possibilities that can cause such a happening. The first is that it's not really a 'red' alga, but very possibly a brownish/rust-looking diatom algae, which can normally appear after one vacuums the substrate, and/or especially in aquariums that do not use completely processed freshwater, i.e., RO 'and' DI water.
As to the quality of the make-up water, tap water, even if processed through an RO system will still contain some silicate (causes diatom alga). Very few RO units are effective at removing silicate. Even quality RO units probably only remove about 90% overall of the in coming minerals and various compounds. Therefore, it's necessary to process the effluent from the RO though a well-engineered DI unit to remove all silicate and remaining elements, one that uses the proper combinations of resins. You may want to visit the SpectraPure brand (see a product review of their unit on my website), which is the brand/model I have used for 'many' years.
As for diatoms on the sand surface, the question sometimes arises why do they keep coming back after cleaning/vacuuming the sandbed. - That's because the colonization of nitrifying bacteria in the upper portions of the sand in the course of their biological processes produce small amounts of compounds containing silicates. In areas such as the bulk water and sandbed interface, these compounds with the "help of light" form diatoms, especially during the enhanced activity period associated with 're-colonizing the disturbed areas.'
Once the bed settles down, and a possible excess silicate situation in the bulk water is resolved, which may take a few months, a conspicuous diatom problem should subside. Also suggest only vacuuming the bed once per month, maybe every 6 weeks on a light bioload tank. If the coating of alga on the sand surface is too heavy, vacuum/siphon out only the upper gains of the bed. This should resolve the problem. Nevertheless, also suggest stopping the addition of iodine for a while to see if that helps, but can say from past experience, excessive amounts has caused unwanted red algae problems in some of my aquariums.
Hope this helps, and keep me posted.