buy calcium nitrate from the same people you get the soluble hydro from or from a landscape supply company. I think it runs about $50 for a 50lb bag. your TDS sounds about right with what I typically run.(620ppm) you don't need to change it weekly. you're dumping to much fertilizer into to ground that way. When the water runs low, check the EC and add water or concentrated solution to bring it back to normal or a balanced solution if it is normal. only change it every two weeks to one month. You can actually run it that way longer if you wish, but then you may start to see more effects from an imbalanced nutrient solution. (I've run them 2+ months that way)
To be truly environmentally friendly you collect your "leftover"'s after each change out in a larger container. After you've accumulated enough (say 10 changes) you have that bulk mixture tested and balance it back to what you need. But for a home grower that is probably a bit much.

It would be a lot easier if your nutrient solution was not exposed to light and air. You don't really know what your plants are using, versus what water you are losing to evaporation and what nutrients you are feeding to algae.

I use Maxibloom from General Hydroponics which comes out to 8 cents a gal when mixed 1 tsp per gal of water.

^^ I have also used the Maxiseries dry nutrients by General Hydroponics. Here is a cucumber grow in a DWC with the Maxi
nutrients ONLY: