Cheato!!!

[Delphinus]

For nutrient removal, it's not so much about how much you have (per se), it's about how much it grows. Ie., it's the growth that causes your NO3 and PO4 to reduce. So pruning is necessary because infinite growth is eventually going to cause an issue with real-estate.

How much of it do you need, well that depends, you can prune it right down to a small amount and it will still come back, but a too small amount will certainly take a while to re-establish itself to a critical mass (ie where the nutrient uptake matches the rate of production).

Short answer is to tame the beast, you have to prune it and dispose (ie., give away, or put in the compost pile or whatever).