Sorry that is not a 20 litre tank it is a 20 gallon long tank. Drew Warwick_d at bls_gov O- OK, let's see. A 20L tank is 5.14 gallons. You have 30 watts of light. That's, um, 5.8 watts per gallon. Bitchin'. Umm, you forgot to mention exactly how many lumens you have over your 20L tank. It sounds like you have a good setup and people would like to duplicate it. How many lumens are required for a 20L tank? George