High Nitrate Levels

[bigfish]

Hi i have a 60 litre hexangnol tank which has various coloured fantails, and for some reason the tank readings for Nitrate tests has been 75mg/litre plus, i do 1 water change a week and a gravel clean but the readings only go down to 25 - 50 mg/litre. My water source is only 10mg/litre so what am i doing wrong?

Any advice would be great.

(tank has fully cycled and been running since Jan 2003)