Oxygen is naturally diluted in water. But how much diluted oxygen will depends on 3 points. First it depends on total width water surface of your aquarium. So it is always better to have wide opening aquarium than tall aquarium. Second, do you have any water movement? Water movement could increase levels of diluted oxygen, because waves and bubbles add more more contact surface to air and attract more oxygen. So it is always better to have bubbles (aerator) if you want your aquarium filled with more oxygen. But make sure to check your fish types. Some fish enjoy water movement a lot. This kind of fish will swim against the stream, and seem to love it. But some fish type like Betta will be stressed if they are kept in moving stream. This stress could lead to death. Betta prefers still water. Third point important for oxygen level in aquarium is aquarium plants. Do you have living plants in your aquarium? more living plants means more oxygen for the fish. Plants release oxygen but it also needs carbon-dioxide to photosynthesis. While water movement and bubbles created more oxygen diluted, it also could remove carbon-dioxide from water. So, having living plants and aerator in the same aquarium will be tricky to manage. I suggest you to choose either living plants or aerator to provide more oxygen for your fish. Or simply choose shorter aquarium with wider opening. Cycle water before putting your fish. Then observe your fish for the first 1 week. If it appear to gasping air along the water surface, it means (always means) it need more oxygen. For quick fix, increase the water movement by placing your filter outlet a little higher (create fountain), or install an aerator. Later, you can put living plants. Too long answer, isn't it? Good luck for your new aquarium. Never give up. Hopefully my answer helps

To some extent yes but if you have a bigger place like pond or unused refrigerator. They will be very happy swimming back and forth. You have a natural oxygen supply from the environment. Especially, if you place some plants like waterlily. They will thrive and multiply better.