3 x 1.5 x.1.5 is around 172litres with 90% water. Gallon is around 45.51.
So you might want to calculate how much wattage ( wpg ).
Different brand have different quality material but mostly the making of the tube are all the same ( correct me im wrong ).
So in the end ,it depends on how much wpg you want eg. 1-2 wpg or 2 to 3 wpg.
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