I was thinking you needed an air-intake stack so that you could suck clean air from over the miasma on the ground. Don't know how to pull it off. Could be natural, do you have "swamp gas", wetlands near that could exhale methane or HS?
Let me know what kind of fish you want. I might be able to put together a box. I'm mainly breeding piranha now, and I know you can't have those. But I have some other breeder sized fish, and it'd just be a matter of conditioning them for a spawn.