Ammonia and nitrite poisoning in low doses won't have a visible effect, but it's likely that it will weaken the fish. The "ill effect" is usually a shortened lifespan.Fishy411 said:i must seem inhumane but i always use fish to cycle my tank. My swordtail has cycled like 3 tanks and will be going for a fourth and he sems to have no ill effects compared to the othr one that i didnt cycle with. At least no ill effects until the other one died.