If you can locate an Asian market pick up some dried Nori. I place the sheets in a gallon sized freezer bag with a slice of bread. The bread "softens" the Nori which you can fold without breaking and placed on a clip in the tank.
Yeah......been doing that for years. I can remember back when I first started going there some 20+ years ago and seeing other LFS owners buying live stock from there to re-sale in their stores.
I have the "in" probe between the out of the RO and the in of the first charcoal filter and I have a reading of 166 while the "out" probe coming out of the di is reading 0.