Ammonia and nitrite are a at zero and have been for been two weeks, but nitrate is still around 40 ppm.The tank has been running for six weeks and all plants and fish are doing very well. Please give answer in ppm

Ideally it is best to keep it under 20 or 15ppm. But your fish should not become stressed. Just try to do a larger water change to get it down. Also test your tap water for nitrates, they may be high. It shocked me when I tested mine and saw that they were high in Nitrates, because I had this same exact problem. I used distilled water until I could afford an RO unit, and it brought the levels right down. BTW I performed two 25% water changes three days apart, then continued with 10% weekly after that. I also cleaned my filter out weekly, and that seemed to help as well. Now when I say I cleaned out the filter, I don't mean changing out all the contents. I just simply rinsed my foam padding in aquarium water in order to keep the Bio filter in tackt. I change my carbon out every eighteen days, this helped me a lot. I hope I helped and good luck, I know how frustrating this can be sometimes.

Nitrate has to be controlled through water changes. Most fish can tolerate 40 ppm fairly well, but I try to keep mine under 20, usually closer to 10. Some places tap water contains nitrate unfortunately, if that is your case, under 30 will probably be the best you can do. Test some tap water before adding it to the tank to find out.

i refuse to give my answer in ppm :O) just kidding ... ideal is under 20 ppm but 40 will not harm the inhabitants ... and if it is a relatively new tank, it takes a bit for them to come down ... keep up with your water changes and all should be fine ...

40 isn't too bad, but don't let it get over 40. ideal is 20 ppm which is what i try and keep my aquarium at. try more water changes.