I do not think anyone can really answer your question with any accurately without knowing what is in your water. At 850 ppm, there is a lot of dissolved solid in the mix already. Is that water coming from a nuclear power plant run off, next to large commercial farm well water, or a large commercial mining operation? On the other hand, is it just a little muddy from some strong rains and no additives have been added like Calcium or Chlorine. GH has a product for hard water, which it recommends for over 200 PPM.
FLORA MICRO® HARDWATER:
If your tap water is over 200 ppm (or contains Calcium above 70 ppm), use Flora Micro Hardwater. If your water contains 30 to 50 ppm Calcium, you can mix our original Flora Micro with Flora Micro Hardwater to create a perfect Micro blend for your plants.
Although, I would not be adding anything until I knew what was in your water. You can call whoever is in charge of your water board to ask them what is in the water or if you were using Well water, then I would have it tested. Either that or get whole house filters or a Reverse Osmosis set up.
As for what your PPM should be using GH, which would also depend on what phase of the grow you are in, the size of your grow, number of plants, etc
For me, I would even want to know the lighting setup because if you have 1000 watt HPS running and it is directly over just a few plants, then you can up your PPM as your plants would be using it up faster than say someone using a couple small CFLs. I like to push my nutes as I am pushing the growth of my plants with my set up. Therefore, I will go higher with my PPM than say a friend who only has a small grow cabinet with say a 250 HPS. Having said that, I still do not push much over 1200 PPM with my soil set up.
