Bob--

A couple of observations. First, the bad news. Grass carp: The second word is an anagram, in my opinion, for their usefulness in most situations (unless you have a lot of bottom-growing weeds, which they will actually thrive on). They will get very large--as in 10-30 lbs (or more in your warm water), and will eat anything ornamental within a foot or two of the pond's edge (except water iris, which happily they leave alone). They will not, however, eat filamentous algae unless they are starving. And you will end up begging your friends with bowfishing equipment to come and find them. Second, the good news. Filamentous algae can be easily removed "mechanically"--i.e., by hand--which can be good fun and the stuff can be used as compost or, if you like the cowpie look, mulch; but even better, chelated copper sulfate products like Cutrine Plus (Tractor Supply, among many other sources) kills the stuff like a hot knife melts butter and has very, very little impact on the aquaculture EXCEPT for adding to the organic load at the bottom of the pond, and has a decent prophylactic effect. The chelated copper will not hurt any fish except grass carp (unless you kill a huge amount of algae, which will drop the dissolved oxygen and cause a kill), and I cannot imagine that the minuscule concentrations necessary to wipe out string algae would affect the water for purposes of watering gardens of any sort. I spray down the margins of my 3/4 ac. pond about 3x a season and use a bottom-diffuser aerator and beneficial microbes, and the results are considered generally acceptable by spouse and friends.