It's easy to make, if you have a KH test kit. Just get a bottle of distilled water, add a very small amount of baking soda to about two cups or so, test it, and just keep diluting it down until you get around 4KH. It doesn't have to be exact, within .5 degree should still work ok.
Did you increase ferts and co2, and reduce your photoperiod? Removing algae won't get rid of it, if the underlying conditions remain. Algae is more of a parasite than a disease, it crops up when conditions are good and dies back when conditions change. It's not a disease that can be wiped out completely.