agreed. nitrates do not rise that high an hour after a water change, even if the gravel and/or filter are gunky. i think perhaps the 10 ppm test wasn't done exactly right, and gave you a false reading.
Welp I did another test its either 10or 20 ppm, although I'm POSITIVE I'm doing the test right, I'm clearly getting differing results, may have the lfs check to see... but yea it's probably false testing
pap... you keep posting 3yrs for api test kits. i may have just thrown out a multitude of tests that had 6-10 months left respectively if 3 yrs is the common life for many of them. know where to find/reference kit age limits? search terms to use or something? i tried looking them up on mfr's sites to no avail. i could kick myself - it was at least a couple dozen kits....
ha! i just wasted a TON of tests that i just knew still worked for no good reason, then. all cause i didn't want to wait for an e-mail response. doh! i even tried to give them to someone who was cycling his tank for the cost of shipping, too. you live and you learn i suppose.
I would look for a source of decay. dirty filters rotting plants, food, ect. I would also use sechem prime. Its detoxifys nitrates. Water changes are not enough. You need to remove the source. Since your tank is new, the sponge or dirty filter media is the only explanation. If you have a good test kit and good tap water. I really like this article. It makes a lot of sense. http://www.bestfish.com/oldtank.html