Reducing Nitrates and Phosphates via Water Change Help

[Stigigemla]

I think it is not a good idea to detail discuss exactly how fast the values will sink.
If the nitrate value has gone from 20 to 40 ppm in 20 weeks the OP adds 1 ppm a week. That means a 10% waterchange every week will in a very long time end in about 10 ppm. But the animals will grow and maybe get more food.
Thats if the current bacteria continues to work in about the same speed as today. I recommended 10% each day so added nitrate via food will make less impact in lowering the nitrate.

Most of the phosphate (98 - 99%) is bound in the rock and a part of it will release as soon as the value in the water is lower.
Thats why waterchanges is not very effective in lowering the phosphate.

In the long term seen the OP has to look over the feeding and probably what animals he keeps.

 

[Randy Holmes-Farley]

As folks have noted, calculators and calculations are totally useless for phosphate reduction by water changes.

Nitrate follows such calculations very well, but phosphate lags way, way behind. As a random example, a 50% change might drop nitrate by 50%, but may only drop phosphate by 5% after the rock re-equilbirates with the water, and really is not a good way to deal with elevated phosphate.

The same is true in reverse for dosing. One can dose 5 ppm nitrate and expect to have 5 ppm bioavailable to organisms. But dosing 0.03 ppm phosphate when it is undetectable to start may give only a small fraction of the phosphate to organisms.