DO NOT go w/ the 14% -- you need a height of 24" for a 14% ---- for a 40 gallon the 12% is a T 5 -- distance is 12-15 inches inside the tank --- 14% bulbs are very strong and require a distance of 24"Should I get a 12% arcadia bulb or 14%? my fixture will be sitting on top of my 40 gallon tank