Hi,
As a rule of thumb, 2 to 3 watts per gallon is good enough for almost any plant. But it also depend on the height of the tank. Which also mean how well can it penetrate to the bottom. 2 to 3 watts is base on an average tank ht. So I assume ht is average for your tank. Arr..do you mean 18watts x 2 tube light hood?
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