I understand the importance of cycling an aquarium. However would it be a problem if I put 3 fancy guppies in a 20 gal tank with nothing else (no plants, no substrate,...). I would use a large sponge filter which has not been cycled. Obviously ammonia would build up with fish waste, uneaten food. But, I would be changing the water out twice per week. So before ammonia can build up, I would already be changing the water. I could see it being a problem if I had say 20 guppies.