That is not too much, I change the water than much in my plant tank.
I have a 350 gallon pond and until this year I would get an an algae bloom too. In the beginning of summer this year I added a Tetra UV 9 watts, my pond has been totally clear since then. I had water lettuce too and when it totally covered the pond I didn't have any algae but you couldn't see the fish through the plants. Last week I hooked up a drip system to one of my large fish tanks. The excess water is drained out side to the pond, it gets about 15 gallons a day. The pond is looking even better with the constant water supply.