Some say you only get 50% from screwins. I tend to think thats a little high. Depending on the type of screw in (spiral vs. u shaped), I would say you lose anywhere from 20-40%. Depends on your reflectors also. You won't find a difinitive number.
A forum community dedicated to aquatic plant owners and enthusiasts. Come join the discussion about collections, displays, tanks, styles, troubleshooting, reviews, accessories, classifieds, and more!