Nitrates... not good

[zxcs]

My fish have been looking a little stressed lately so I decided to do some water tests. Turns out I have 125mg/l nitrates in the tank. I quickly rush and do a 30% water change with RO water from my RO unit....
I test the RO water and it has 12.5 (NOT 125) mg/l nitrates. For some reason my tap water is high in nitrates and it runs the filters on the RO unit down fast (the two "cheaper" filters, not the resin one have gone green) so I constantly need to keep buying them. I have two questions:
Firstly, do I need to change the resin filter on the RO unit? Is that what is letting the nitrates through out is it the two other filers (which look like foam and sponge)?
Secondly, how can I remove more nitrates from the tank? I have a nitrate filter sponge in the filter unit which I changed today and I've dont a 30% water change but still have nitrates. Any suggestions?
Thanks!

 

[scsinet]

Are you 100% certain that your test kit is not faulty? I'd suggest taking a sample of your RODI unit to your LFS and having them confirm your results.
Secondly, the two cartridges that you need to look at in your RODI system are your carbon cartridge and your RODI membrane. Are you testing your RODI with a TDS meter? Don't feel bad if you aren't, I don't either. You may want to test your water with a TDS meter to help confirm your results.
Next, start looking at what you are doing:
1. You feeding... how much/how often
2. Water changes.. how much/how often
One thing you can use to naturally help with nitrates is a vegtable filter, aka a refugium, but it won't help with a problem as severe as you have.