20 is fine for nitrates. Do you have any corals or inverts? I would keep it 20 or below if you have corals or inverts, but if you have fish only, 20 isn't going to do anything. Nitrates at 20 ppm is not worth doing giant water changes every day. That will be more stressful to the animals than the nitrates themselves.
There is filter media that removes nitrates (Kent Marine and Seachem both make one), it looks like white carbon. But that takes several weeks of use before it kicks in and makes a difference.
Chaeto is a type of macroalgae -- chaetomorpha. Again, adding macroalgae isn't going to lower your nitrates overnight either.
The worst thing you're going to see happen with nitrates at 20 is MAYBE a little bit of an algae bloom. And even so, 20 isn't high enough to create a big algae problem.