A brother of mine had two fish tanks 25 or 30 years ago. Guppies, mollies, platies, swordtails, zebra danios, angelfish, are some of the species he kept at that time. I never touched the tank tough. I used to believe you had to change all the water in the tank when the time came, but that it’s not true . The tank its been cycling for a week now, and the second day I added 3 zebra danios, but the levels of ammonia are still untraceable. Nitrites low too. The tank is 36 gallon capacity, so I decided to add 4 guppies(3 female fancy guppies), and a turquoise male guppy. One female seems to be pregnant(who knows the father? I don’t) The male was bought at a different store, so they are not related. Now more poop will increase the potential to create more beneficial bacteria. In the process new fry guppies will come up from those mother bellies factories.