Nitrate 0 ?

Ummm. As you know Aquamaid I am VERY new but I thought there had to be some nitrates because of what is in the tap water.
Probably wrong, knowing me!

Richard posted while I was typing 

What is your tap water nitrate? Most people who have high nitrate find it comes from their water supply.

The nitrate test is the most inaccurate one of those we use. When my son worked for the water testing company he was very amused at the idea of us trying to test for it in the home with little bottles. To get an accurate reading, you need very expensive lab equipment.

Is the tank in question the 232 litre with 9 congos and 12 harlies? They are not going to make much ammonia and it will be very diluted in that sized tank. According to the CC, you are only 24% stocked. So if your tap nitrate is low, the fish won't be adding much to it. And you do have plants, though I notice you  have been commenting on them! Plants will use ammonia as food stopping it getting turned into nitrate, and they will also use nitrate from tapwater.

And if you are still worried, you could get it tested at a shop. Or several shops to get an average.

In that case, I would suggest that your plants are using up the nitrates in your tap water. And although you are doing water changes to keep your tank water in good condition, your plants are actually improving the quality of it!  Keeping an eye on how the nitrate levels in the tank fluctuates will help you work out how much water to change and how often. I would suggest doing a water change when the nitrate level drops to about 5ppm. And I think that for a planted tank, smaller water changes are better. 25% at most, but 10% should be fine.

Having said that, there will always be occasions when a big water change is required - eg after a death. Because I've learned that my planted tank now has quite different parameters to my tap water, I do quite a large water change prior to introducing new fish to try and minimise the sudden change for them.