Where is the nitrate?
 

I have a 37g tank, unplanted, 5 zebra danios, 4 panda cory catfish (more of both on there away). 8 days ago i checked the parameters and they were pH=7.5 Amonia=0ppm nitrite=0ppm nitrate=3ppm. The next day I added the four pandas.4 days later i vacuumed and did a 20% water change. When I checked the water tonight the parameters where 7.5/0/0/0. I know that the amonia and nitrite should be zero but is it ok that the nitrate is also 0 (the water change was two days ago)?

Hi I am no expert but I did read up somewhere that bacteria in substrate could turn nitrate into nitrogen gas and escape out the surface of water this way! I'm not sure in what quantities or anything but I bet some other folks around here would be able to answer that :)

Leave it a few days to a week and test again....this should give you a reading...

Also make sure that your test kit has not expired..this can give false readings.

It is possible that you have a perfectly balanced system and there is indeed no nitrates being produced with your water changes etc. Try leaving the tank untested and no water changes and see if you get a reading.

If you get a reading then return back to your regular schedule but perform a slightly larger water change.

Tests should be done about 24 hours after a water change to allow the system to settle back down.

Thanks for the advise, I am not too woried as the fish seem happy :-) I will test again in a few days.

Usually it is Methane gas that is trapped in the substrate then released when vacuuming, this is caused by the Anaerobic bacteria in the substrate. Usually a good sign of over feeding if you see bubbles coming up when vacuuming.

Most will never cause problems unless allowed to build up to excess levels, which will be noticed by other means before they cause any harm, ie stressed fish and readings of ammonia.

so... the nitrate is still reading zero, maybe one.

Just out of curiosity how long has the tank been cycled for?

The original tank was up for three months, the new tank (containing everything from the old plus more) for about a month.
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Well being that your tank wasn't cycled for long (not saying it wasn't cycled but 3 months isn't that long). It is possible that by siphoning the gravel and adding fish you could have disturbed the cycle. If it was me I would test the water everyday. Your tank could go into a mini cycle. Now I might be completely wrong here but that's just my opinion.

i will continue to test, but every test in the last 2 weeks has had 0ppm for both ammonia and nitrite though.