80+ppm is considered on the high side, while most hardy fishes and shrimp can tolerate those levels for a period of time, its still a good idea to reduce it and keep it at more moderate levels of below <40ppm.
Most likely the plants (if any) in your tank are just not consuming and removing the nitrates as fast as its produced, or the bio-load is too high, hence the accumulation over time.
Increasing the density of fast growing plants, reducing bio-load, cleaning the filter more frequently and doing more water changes will help keep the nitrates in check.
Since your tap water already has 20+ppm nitrates (which is also considered high for tap water, ideally it should be zero or at least at lower levels, might have to check your water source or pipings), your current water changes will have less effect on reducing nitrate levels. So either have to do more frequent water changes to compensate, or switch to distilled water, or purify the tap water using an RO/DI unit before using.
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