You cannot catch fish in your toilet. The level of water is too low and the concentration of sewage is too high. Fish need oxygenated water to survive (it's how they breathe), and toilet water isn't oxygenated enough. Even if it was, the level of sewage would kill any species of fish. Toilet water goes through a reclamation process that filters out all sewage and bacteria before becoming "grey water". Grey water is then often used for irrigation or other non-consumption purposes.