Nitrates come from something. Usually it is due to overfeeding or high amounts of waste coming from the fish which is due to overfeeding. Two other things are something dead in the tank or unpurified water added in the tank whether it be top off water or using unpurified water during water changes. I was maintaining a Dr's tank once and I had the nitrates at 0. The person who was doing the feeding was on vacation and on my next visit out, i found the nitrates at 25mg/l. I asked who was feeding and the dr said he was but very very little. Of course i didnt believe him cause everything else checked out just fine and I knew the water I use is purified. So I told him, "look doc, there are only 2 things in your tank that would cause it to rise, something dead / or overfeeding and every fish is accounted for so that leaves one thing" he said no he wasnt.....I said " okay, your a dentist and you see a kids mouth full of cavities and plaque everywhere and you ask him if he brushes and he says "YES, in fact twice a day" do you beleive him knowing that plaque builds up due to not brushing and eating lots of candy. He said "no" and i said, "well in this case, i am the dentist and you are that kid." he laughed and said he in fact fed them more cause they looked hungry all the time. I told him "theytrained you. what other way are the supposed to look"
