When your Ppm hits 1200, is that the measurement after the resivoir level drops or after it's been topped off with water? It could be from the plants being extra thirsty and just drinking water without touching the food. If that's the case, top off with straight water, then measure ppm. Then measure ph , add nutes if needed, then adjust ph of resivoir as needed. Unless I'm having root problems I only change out resivoirs once a grow between week 1 and 4 of bloom. Changing out your res means all the bennies that were growing in it get wiped out. High ppm just needs to be deluted with plain water...
I use a submersible pump with a hose attached. It gets dropped into re resivoir, and pumped to 5 gallon buckets outside of the tent. Yup exotic seeds did a wonderful job with the tangerine kush. I also started with great white and switched to orca. I won't grow hydro without orca. It flat out rocks. I've also tried botanicare hydro guard. Not near as good as orca. I'll easily get a years worth of bennies out of the 40 dollar bottle of orca ha ha. Yes I top soon after switching to bloom. It stunts growth and helps tame stretch. I very rarely get more than 6 to 12 inches of vertical growth after I top them. In a small tent with high powered lights it's a nessicity ad I need at least 12 to 18 inches of clearance from bulb of 750w DE hps to canopy. Otherwise I'll burn the shit outta the plants and light stress them. This is also strain dependent. Tangerine kush will take 750 at 18 to 12 inches. Other plants need 18 inches at least, and need to be acclimated to that kind of intensity very slowly. Light stress is a very real issue that will trigger hermafrodite responses from sensitive plants. Ask me how I know ha ha.
Oops another rabbit track. ADD is a bitch. Anyhow I firmly believe in order to know what your plants can take, you have to push them till they tell you to stop. Once you know what they can handle, act accordingly.