If you want sheer volume, you might want to consider some of the barbs.. like Odyessa barbs, rosey barbs, etc. Get a species where the males and females are differently colored, so you can keep them seperate (when you fatten them up for breeding)
A 20 long with egg crate in the bottom, put the males and females in together, remove after 24 hours. You get a lot of fry. Sure, it's a PITA to raise them up, but you can get a lot of volume this way, from maybe 6-8 starting fish. Once you get them eating flake (pretty quick) and move them to a large growout tank, they grow pretty quick.
Blue Gourami might also be a good idea. They can lay like 10,000 eggs at once (IIRC). Again, it's a pain to raise the fry to flake eating size, but that's a lot of fry from one mating.