WoW thats more like a vase than a tank (no malice intended)
The usual config for a tank of those dims would be 12" long x 7" wide x 7" deep.
there is more surface area.... What did you use that tank for.
The only killifish that I would place in that tank and it would be only as a short term breeding setup would be Ps. annulatus (clown killi)
I think it would be wrong of me to suggest otherwise.
Regards
BigC