20-40 is a pretty broad range. Can you tell from the color what the reading is closest to?
Nitrates are something plants actually use, so it's better if you do have some
in a planted tank. That reading should not do any damage to either your plants or fish. If it climbs higher, though, do a PWC
I am moderately planted and my nitrates stay between 10-20 ppm
1 yoyo loach, 2 polkadot loaches, 6 cherry barbs, 2 turquoise rainbows, 1 chinese algae eater, 2 harlequin rasboras; java fern, java moss, hornwort, Asian ambulia