| | If we could start over
Pan. As I understand it, Red algaes are mostly marine, having evolved in that environment. There are fresh water types too though, like staghorn, bba, brush algae, too if I'm not mistaken. They only show red with alcohol prep.
Excuse the use of common names, which we usually have a fit over when refering to (higher) plants, but the Latin for these algae is seldom seen for some reason. Perhaps this is because no one is really sure of applying the correct taxonomy in the appropriate places. I'm certainly not.
This point about the CO2 may be the key because, now that I think of it, the staghorn's re-appearance after a long nap was concurrent with my running out of CO2 and not replacing the tank for about a month. As I recall, I was busy and also curious to see the effects without it. Guess I have my curiousity satisfied now!