I personally prefer Cannazym for roots but if they get bad I'd switch to H202 at 2 teaspons/gal every other day of the store bought 3% or the hydro type (35%) at 2 ml/gallon every other day.. You can't mix peroxide with a lot of the zyme products....but I think you can still use your SM 90...you might doublecheck that.

You need areation to your DWC to keep roots healthy....otherwise they will suffocate and drown. Do you have a photo of your setup and roots?

PS...I use DWC for my plants and they do well as long as the roots have enough room and the areation is good.