A nitragone would be your best bet. £50-60.
www.romford-reef.co.uk/main1.htm
check under equipment/nitragone as the link only take you to the site not the page needed to view it. Its not great,but dose the job. takes a few hours to clean/filter 50g. can recharge thousands of times in dish washer salt/last many yrs. cheap to recharge and it will remove 20pph from your tap water giving you almost 0pph as long as you have the time to filter the water through it each water change. then flush it with salt for next water change to recharge.
RO waste to much water if you are on a meter(no use cos to expensive)
