usually use it before a water change. doesnt effect water chemistry and helps get all small particles ready to be sucked out leaving the water crystal clear. 2-3 drops per gallon (I use 2/g). It also somewhat cleans the tubes of filters, so expect to clean those out as well.
E is for ponds or if you just have alot of tanks and I think its measured in caps/gallons. "Clears your pond of suspended algae and other particulate matter causing particles to sink to the bottom. For best results, use a pond vacuum to clean the bottom after treatment."