well, I have a 250 gl tank filled with carnivourous fish and my nitrate levels rise almost 20 ppm weekly. My other 50 gl. tank, wich is planted, with regular fish, takes 3 weeks to rise 10ppm of nitrates.budrecki said:I have a 180g tank. do you know how long it would take for nitrates to reach 80? My nitrates increase about 5 between weekly water changes of 50%. Do the math, thats ALOT of weeks without water changes.