See if they have the 10lb box--the smallest amount. It will last years. Backwash then start with about 1/4 cup. Wait 20 min to 30 min and see if your pump pressure goes up 1 lb. If not, repeat with about 1/2 that amount. When the pressure has gone up 1lb, you'll know how much to use. If you put in too much, your pressure will shoot up and your flow to your skimmer and return will noticeably decrease. You'll have to backwash again to clear it out, then start again.

I don't think it adds anything during an algae fight--you don't need the filtering to be that fine until AFTER you've won the battle as the pressure will be rising every day from just the algae in the filter that you should be backwashing out everyday during the fight. After, the DE will help 'polish' your water till it looks crystal clear.

DE and skimmer socks are two of the cheapest, best additions to help keep your water beautiful.