05-05-2010, 08:11 AM
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Surface algae, like what you have, can cause several detrimental effects. Firstly, it covers the surface of the water, thereby blocking effective gas exchange. Second, it absorbs most of the useful light, thereby blocking it from your submerged plants. Third, it will cover and smother above surface plants.
If it's under control, that is, only covering a small portion of the surface, then it won't cause any noticeable damage. However, if it gets out of control, you'll start to see problems.
Incidentally, some surface "sheet" algae is actually more akin to cyanobacteria, but the effects are the same. The only critical difference is if you're going to use chemical means of control. True algae needs one method, cyanobacteria needs another. But, I usually recommend against chemicals in the tank, so, for me, that's a moot point.