Water testing questions - in no particular order

First - is 40 ppm nitrate a problem? All the sources Im seeing say "no problem up to 40 ppm." Well. Im at 40 ppm in the indoor tank so I can only surmise that its not good and ... fish are seeming somewhat lethargic, not their usual selves. Part of the problem is that water out of the tap is ranging from 10-20 ppm, which doesnt give a lot of leeway to accommodate additional nitrate from fish waste. This indoor tank is 40% covered by hornwort so Im not sure what else Im supposed to do. besides a big water change, and I do mean "big" because to affect a change when you already have 10-20 ppm it will have to be a lot of water.

How do yall cope with nitrate in your water - are there filters that exist?

Ammonia tests - Ive tried 3 different ones already and all have the same problem - the charts that are provided to compare your sample are all so poorly printed that the color of the chart doesnt look ANYTHING like the color of the sample, If they can print fairly accurate paint color samples we should be able to get a decent chart that actually relates to the colors of the samples. Having to guess or come up with a range of numbers that indicate what it MIGHT be is not very useful.

The tests which color the water in a test tube are not very useful for pond water which is already green from algae - the same shade of green which indicates ammonia!

The API ammonia test tabs have been coming out yellow with a dark green edge around it, and the Seachem test with the little round dots are blotchy green and yellow - how the heck am I supposed to read those???

The Seachem test is supposed to be able to test for free (ie bad) ammonia only - however when I tested water out of the tap plus a dechlorinator that has an ammonia neutralizer in it (Stresscoat) it tested positive for ammonia! Plain tap water - no Stresscoat added - tested negative. So apparently it DOES give a false positives�.