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I had no top on my 56 gallon and lost around 3 gallons a weak. I have a top on it now and loose only 3% of that now. I also had problems with insects landing in my tank and the top helped 100%.
I go through around 5-7 gallons a day on a 220 gallon system. Ro/di unit has more than paid for itself at this point. Great purchase if you are considering one.