enrique? where are you located? San Antonio tap water is not condusive to an algae free aquarium, I tested my own tap and it has nitrates in it. I would use RO water or buy ro or distilled if you can. How old is your tank? every tank has an algae phase it goes through.

If you don't allready have it, you should get a few detrivours and sand sifters, the fact that you say the algae is growing on the gravel tells me it's not getting turned by a sand sifter, grab some nassarius snails, and maybe get a live sand culture from folks that have mini-stars and pods and what not...

Hope that helps, any more info you can give us on your tank will help.