Are you wanting to use this tank for the adults to live and spawn? Will you have separate tanks for holding females and fry, or are you going to let them spit in the tank? Are you planning on selling the resulting fry? If so, I'd pick a species that isn't very common, and that you have guaranteed homes for, such as supplying an LFS, etc.
To get the most fry out of them, you'll want to keep the adults and fry separate, ideally you'll want at least 3 tanks, the spawning tank (the 40gal), a tank for holding females 10 gallons or more, and at least one fry grow out tank, preferably 20 gallons or more. In the holding tank and fry grow out tank(s), sponge filters would be best, in the adult spawning tank a cannister filter will be fine.