You would need a 20" fixture --- Zoo Med would be the way to go but I recommend for a 40+ gallon tank and a height of 18" go w/ a 24" fixture w/ a Reptisun 10.0 T 5 bulb --- that way if you decide to upgrade to a bigger tank the 24" fixture will go right into the new tank --- you can also go w/ the Arcadia Pro T5 --- you would need the Arcadia 12% bulb that would come w/ that fixture -- they are another great bulb ---- this site has the both the fixtures but is currently sold out of the Zoo Med https://www.pangeareptile.com/store/arcadia-prot5-uvb-kit.html