first things first, test your replacement water before actually changing it!!! highly unlikely its the test kits, there maybe a little difference between kits but not enough to detect 0 vs 25 for eg. as reef says, provide more details about your tank, eg: stocking levels, skimmer make/model (vs the 600 litres you have), kg of LR (for biological filtration), feeding? etc etc. remember reducing nitrates wont happen overnight,... but it will happen (go the shampoo ad)