OK, I thought you might be in coco. When was the last time you dumped your reservoir? I assume you are changing out to fresh nutrient solution at least every two weeks, topping up in-between?
I guess it would be handy to know the rest of your set-up. Like, are you using large rockwool cubes in a flood and drain or NFT system? In which case, simply dump your res, flush the rockwool with plain (pH adjusted) water, then mix a new batch of nutrient.
As to what strength nutrient to use, firstly, how do you think you ended up with excess nutrient to begin with? Are you not changing out you res often enough? Do you think you are adding too much nutrient in-between res changes? Are you using bottle-strength nutrients that possibly your strain can't handle? Additives? Are you keeping an eye on your EC/TDS. Do you have a lot of rockwool exposed to the air that could be drying out and building up salt concentrations because it is not being flushed by design?
There are obviously quite a number of possibilities, but you're best-place to know what you think the issue might be.