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We have had our reef tank for about a month and a couple of weeks. For the last two weeks we get red algae on our live sand; after we remove them, they reappear after less than two hours. Attached photographs show how does it look like.

How do we remove them forever?

Please note that all our corals do very well and all water tests are OK.

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That's not algae, those are diatoms. That is part of the natural cycling process and are usually an indicator of high bacteria if you are dosing carbon (it's feeding on the excess bacteria) or disoolved solids (TDS) in your water.

If you are dosing carbon, cut it back slowly (or if using bio-pellets, you might be using too much). For a tank that's not mature, any carbon doing should be minimal.

You should also take a close look at the water quality you're adding to the tank for water changes and top-off. Anything above 0 TDS will provide food for the diatoms. Make sure your are using RO water that is pure (run through DI) and the DI is not spent.

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    diatoms are a major group of algae, so your initial correction isnt entirely correct in itself.
    – James T
    Commented Oct 12, 2015 at 14:32
  • @MarkS we buy water from our fish store and the tests show everything equal to 0. In our tank we have a very little amount of nitrates, but we didn't have any carbone dozing at all until a week ago when our skimmer was delivered.
    – Diana R
    Commented Oct 12, 2015 at 14:40
  • @James Trotter, you are correct, I understood her mistakenly understood the question more of a the traditional algae types. Diatoms are a planktonic type of algae.
    – Mark S
    Commented Oct 12, 2015 at 18:26
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    PO4 tests aren't 100% accurate because the algae, cyano, or diatoms will be consuming it resulting in a falsely low reading. The fact that these exist in the tank, is an immediate tell that there is excess PO4 available. I would almost guarantee this is the result of an imbalance between available NO3 and PO4. The pellets reduce the nO3 but aren't good at reducing PO4. I've never not had to use GFO or another PO4 media when using pellets or carbon dosing.
    – Jestep
    Commented Dec 11, 2015 at 17:35
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    Also, almost 100% sure this is cyanobacteria and not diatoms.
    – Jestep
    Commented Dec 11, 2015 at 17:38

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