What happened is the first water change removed 40% of the nitrates. Leaving
60% in the water. The second water change removed 20% of those nitrates, not
another 20% of the original level. So even though you have changed 60 % of
the water, you only got 52% of the nitrates.

If possible, changing aboout 70% and then another 70% will drop the levels
significantly. That should drop your nitrates by about 90% from the current
level

Are you using R/O water? The tapwater in my neighborhood has low levels of
nitrates in it. Over time, this built up in my tank.

Another thing I found way back when was a waste buildup in the hose leading
down to my filter under the tank.
"SCAFFMAN" wrote in message
. uk...
i have a 90gal tank

of late the nitrite has been very high,

even after a 40% water change and a 20% water change a week later

i havent lost any fish of late and have reduced the food, and missing
feeding on some days.

any pointers



thanks