I had similar problems in my old 20 gallon. In tanks that small, there's just not enough water volume to dilute the small amount of nutrients you're adding through even sparse feedings between water changes, especially if there's no skimmer or other method of removing them as they're produced (other than the algae of course). I'd be willing to bet that Prodibio won't totally solve the problem, since I bet even the tiny amount of nitrate/phosphate produced per feeding is enough to keep the algae going. Even the most Spartan feeding regime in a tank that small can lead to severe eutrophication in a matter of days.

I know what I'm about to say is complete reef heresy, but I think you should explore how feasible it would be to do much larger water changes, as I bet that would solve your problem. I set up a 5 gallon pico tank to tide me over while my new one is being built following the 'eco-reef one' philosophy (http://reefbuilders.com/2010/04/22/e...ano-reef-tank/) and while I thought 100% water changes were nuts at first, it's so far blown my more traditional 20 gallon nano out of the water in terms of coral performance. Other than a brief diatom bloom, there's been zero issues with algae, and I'm feeding all the corals pretty aggressively, getting one syringe full of Acans plus early in the week, and a mysis shrimp per polyp later in the week.

doing a 100% water change on a 12 gallon would be way more of a chore than a tiny 5 gallon, but maybe if you did it one out of every two or three water changes, you'd progressively deplete any built up crud in the tank that's adding excess nutrients.

And no, so far I haven't seen any negative side effects on the corals from doing 100% water changes (with really close attention paid to matching temp and salinity). Within an hour they're all as inflated and happy as they were before I started.