First, you try to remove as much of it as you can with a toilet bowl brush or some similar type tool. Then you buy the regular 3% solution sold in those brown bottles at the pharmacy.
The common figure you hear is 1 pint per 1,000 gallons of pond water. Pour right on top of the alagae--not the fish. You should see the algae start to turn white or brown within the next day. So get ready, because you do need to remove this dead stuff immediately as it can quickly clog everything in site. You need to clean your filter like crazy, too, until it's all gone.
The biggest potential problem is that if you have massive amounts of string algae you can create a very large oxygen demand so that's why physically removing as much as possible before doing this is critical. It may be necessary to have an aerator ready to put in the water. Adding hydrogen peroxide, while effective in removing algae, will essentially result in re-dissolving the nutrients which contributed to the algae growth to begin with. It's sort of a vicious circle sometimes.
I remember DoDad saying he used a 27% solution, but can't recall what his ratio was or his results. Hopefully he'll pipe up to recount his experience.
