LF 10-15g cube tank or other (GOT ONE)

[mws]

My 11year old wants to try her own little saltwater tank, with my help of coarse. She had a freshwater for a year and a half.
So looking for a 10-15gal nano, cube preferred but would take any tank what is not more then 20" long as I will let her use my old current usa 6 bulb fixture, what is 20"long.
Let me know what you have and price.
8-12 gal biocide or similar is also a possibility but will only pay $100 for such tank.