40 of nitrates generally won't pass municipal water quality standards ... Unless you have well water, I would be a bit suspicious of the 40 ppm result.
The nitrate test is rather tricky to do. You have to follow the test method exactingly ... esp. with the shaking, etc. Bottle #2 is an emulsion, and if you don't shake it enough, you get some wonky result.
So, the first thing to do is to validate your test. Get some distilled water & test that, you should get zero. If not, you have a bad test kit or bad technique. <Incidentally, if you have done a few tests without proper shaking, the concentration of reagent in the bottles will no longer be right, and the kit is toast ... Happened to me before, have to go buy new reagents.>
If you have some nitrate (like KNO3 fertilizer if you are doing plants), you can make a known concentration of NO3 with distilled water. Test that & note what the color change is. That gives you a far better idea of what the color change is supposed to be, rather than relying on the card. If you don't have nitrate to play with, then do the serial dilution as outlined above. You should see the nitrate level decreasing with your dilution. If the dilution doesn't reduce the nitrate level (or if the stock test solution tested way off), again your test kit is toast.
Finally, if you have municipal water, try to get a water quality report. <My water co post the daily test report on-line.> That should tell you how much nitrate is supposed to be in the water.
If you really have 40 of nitrate in your water, you will need to doctor the water. A RO/DI unit is prob best for big tanks. For small tanks, you may consider getting RO water from the grocery store, or using bottled water. You can either get straight RO & mix in an appropriate FW buffer salt, or mix in some tap with the RO to get an appropriate buffering capacity. <Exactly what is best depends on what actually is in your tap water.>
