I would do two 20 watts one in the front half of the Viv & the other towards the rear so that you only leave 7" or so on either side. That should be enough for plants with a medium to high light requirement, but seeing as that is a nice long & deep Vivarium, maybe grab a small 14 watt LED since shipping would be free at that point anyway & that would allow you to really play around with it, better more light then less if growth is the priority. depending on where you plant everything you really should be fine with just the two 20s & IM only suggesting the one 14 watt as a backup option bc shipping would also be free & they're really great bulbs for the 15$ sale price.. If you end up not using it you can save it for a smaller tank in the future, I use the 14 watts on my 12X18s expos & 10 vertsand they give me great growth. MY paypal is
idris2279@yahoo.com , let me know what you decide & please leave your shipping address in the notes if its different then your paypal address… THanks in advance for the interest as well, Idris-