Fisn In Tank Cycling
Using fish to cycle a tank is fine. You do need to use very hardy fish. This is where many fail. If you like livebearing fish Guppies, Swordtails or Platys will easily survive the nitrogen cycle. Some hardy egglayers are Rasboras, Rosy Barbs, White Clouds and Zebra Danios. I used 3 to 4 small fish for every 10 gallons of water. Once the fish are in, you wait a day or two for oxygen to mix with the ammonia from the fish wastes. Use your water testing kit to test every day for traces ammonia or nitrite. When you have a positive test, remove about one quarter of the water and replace it with pure, treated tap water. You're keeping the fish safe and also growing your good bacteria. Just feed the fish a tiny bit every couple of days.
Test daily and remove the water when needed. When several daily tests show no traces of ammonia or nitrite, the tank is cycled.
I'd add a good floating plant like Hornwort. Just drop in some individual stems. The plant will help use the wastes.
"Fear not, my young apprentice. Just change the tank water."