Agree with Boondoggle. When you rake out the algal mats, they must be removed from the watershed area where surface water goes back into the pond. Otherwise, the algae breaks down and the nutrients go right back into the pond!

The water from your spring probably has a slight fertilizing effect on your pond. Calcium, iron, plus many of the other trace metals support plant growth. (Of course, N-P-K are the most important components of pond fertilization.)

Finally, I don't believe carp eat more than a tiny amount of FA. Have your read any of the threads on tilapia? They will eat your algae! I believe you are far enough south that they will probably live through your winter temperatures. (Maybe check with the other Texas pond owners that have them?)