I have read on numerous sites you have to zone for high tech industry. My question is what zone to use the medium-density industrial or something else?
There is no dedicated high tech zone in the game, all industry (except agriculture) uses both medium and high density industral zones. If there's enough demand for I-HT, those buildings will grow. When you have a solid I-D and I-M industry already, you can try rising the taxes for those a tiny bit, so the demand for them will lower, hence only I-HT will grow.
Or you can use T-Wrecks' "Industrial Revolution Mod" (IRM), which is based on a suggestion that I once made. It restricts all I-D and most I-M lots to medium density zones and moves all I-HT lots into high density, so when you zone the appropriate density, only the desired buildings will grow.
Thanks