not to mention (not thinking this would effect at all) but if it were to red tides are a phytoplankton bloom ...live pod/coral food
In the case of red tides, they are what is a called a Harmful Algae Bloom (HAB). Not something you want anyplace, tank or nature. That said, not all algae blooms are of harmful algae, and the red tide algae is not the only harmful algae. So lots of variability. The big key with rainwater induced algae blooms is not so much the fresh water input, but the nutrients that come along with it...runoff from the land and nitrogen in the rain itself.