Your local hydro shop probably won't have slow release fertilizers because they are not hydro nutes. They are meant for soil. Any greenhouse or nursery or hardware store should still have them somewhere. And if you do decide to stick with slow release nutes, only use those, not those and hydro nutes (way too much).

As far as guessing how much more stretch is going to happen, I don't know...how old are they?

SMG