Also, keep in mind that in theory, nutrient reduction should be proportionally reduced by the amount of a water changed, assuming you aren't adding nutrients. In a tank that size, water changes are probably cumbersome, but look at these numbers for a minute:
100g change (20%) - Reduction 2.50->2.00
100g change (20%) - Reduction 2.00->1.60
With 2 water changes, it becomes much more manageable. Just remember, the bigger the water change, the more significant the impact.
250g change (50%) - Reduction 2.50->1.25
These numbers are accurate in a perfect world, but you should see a pretty significant change with a few big water changes even if they're not exact.
Also... double check your meter to be sure it's accurate.