When setting up a refugium, it is usually for the purpose of growing macro algae to absorb nutrients (lowering nitrates) plus photosynthesis provides oxygen and depletes CO2. Cheatomorpha is a great macro to use, because it doesn't have a problem of 'going sexual' and grows nicely. If you can't buy Caulerpa, this ia great alternative. Even if you have Caulerpa, you might consider switching to this as I have. Ignore the crab in this picture and focus on the green stuff. :)