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From your question, I'm not sure how large your aquariums are. The all in one tanks are usually on the small side, so you have to be very careful about regular maintenance and not over crowding. During the nitrogen cycle process, the aquarium should start to develop beneficial bacteria. If you have access to an established aquarium, a cup of gravel from it will help the bacteria, as will live plants and a small pinch of flake food every other day. Some aquarium shops even have starter bacteria for sale. Without the bacteria, waste materials will turn into ammonia, which is toxic to fish. While the bacteria colony is getting established, the aquarium may have ammonia spikes; the cycle is considered complete when you have no ammonia spikes (zero ammonia) for a couple of days. You can buy a reagent type ammonia test kit for less than ten dollars, and the fish store may test your water for free. Please don't add fish until the nitrogen cycle is complete. Depending on aquarium size, cycling can take several weeks. While you're waiting, you can do a little research on the habits, requirements and adult size of the fish you want. It's good to begin with cheap, easy fish and plants, because losses are common when you're getting started in the hobby. It's difficult to resist over feeding, which amounts to killing the fish with kindness.
Reading through some of the postings and blogs on this website will increase your aquarium keeping skills, and we're always happy to answer your questions.