18" bulbs are actually 15watt, the 24" is 20watt. The cheapest lighing upgrade atm, would be a 48" shoplight with 2 32watt 6500k T8 bulbs. Probably cost no more than $20 with tax at home depot or Walmart.
Crypts, Anubias and Jave Fern would be the optimal plants at that lighting level and...