If you ever think your nitrate is abnormally low, go back and check ammonia and nitrite. They should always be 0. You can test for nitrate in tap water with the same test. Most people don't have that issue, but its important to know if you do.
You have a lot of fish in that tank, but they are all small ones. If you feed lightly and your nitrates take awhile to go up, you can go further between water changes than most people. It really isn't any harder to hold nitrates at 10 or 20 than to hold them at 40. What is important is that you know what you need to do to keep them under control.
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