What is better, nitrite or nitrate for aquariums?

Between nitrite and nitrate, nitrate is the one you can live with. Nitrite is straight-up dangerous in a fish tank, while nitrate is the end product of the nitrogen cycle and only becomes a problem when it builds up too high. Nitrite (NO2-) is toxic because it messes with how fish carry oxygen in their blood (you will sometimes hear it compared to "brown blood disease"). In most community tanks you want nitrite at 0 ppm all the time. If you test and see anything above 0.25 ppm, treat it like an emergency: do a big water change (50% is a common move), stop or reduce feeding for a day or two, and make sure your filter is running properly. Adding an extra airstone can help because stressed fish need more oxygen. If you use salt in freshwater tanks, a small amount of chloride can reduce nitrite uptake, but water changes and fixing the biofilter are the real solution. Nitrate (NO3-) is much less toxic. Ideally you keep it low, but a reading above 0 does not mean your tank is crashing. For many freshwater community tanks, keeping nitrate under 20-40 ppm is a solid target. Sensitive fish (some shrimp, certain breeders, and fry) do better under 10-20 ppm. Saltwater reefs usually aim lower, often under 5-10 ppm depending on your setup. If nitrate is high, the fix is usually simple: larger or more frequent water changes (like 25-50% weekly), less overfeeding, vacuuming mulm from the substrate, and adding fast-growing plants if you keep freshwater. So if you are choosing which one you would rather see on a test kit: nitrate, hands down. Nitrite should be zero, and if it is not, it usually means the tank is still cycling, the filter bacteria took a hit (like after replacing too much media or a medication), or the tank got overloaded too fast. A good routine is testing weekly in a newer tank, then every couple weeks once things are stable, and using nitrate as your guide for how often to do water changes.