what's the size and volume of your pond? temperature? you realize by constantly changing the water you are just opening up more volume/resources for the algae to continually take over, right?
would be best to leave the pond alone for a bit and let the algae die off after it consumes the nutrients...you could also go with some other plants. the gar should be fine with excess algae, but we don't know what other fishes you have in there.
i've used algaecides with tanks before and they hardly ever work; didn't seem to bother the gars much--
--solomon