Here's some random thoughts in no particular order:
Glad to see someone else using CNS 17 Coco/Soil. I've been to half a dozen shops in my area and not a single one of them carries the Coco/Soil formula. They do have the Hydro one, but not much of it. When I ask about it they all say it's because they "sell so little of it" and I'm like... yeah, because you don't have any.
I think it's because the markup on it is so minimal. A quart of the stuff can be found online (before shipping) for about $9. Gallons for $20. It's just such an inexpensive and simple system that is so perfectly dialed for cannabis in coco I can't understand why more people haven't at least given it a shot.
In your particular situation, with the hard water, you'd actually be better off with the Hydro formula as it contains less Calcium. The Coco needs a good deal of Ca for a while but later on during flowering this can become an issue.
The ability to test your EC will allow you to compensate though. In addition, if a RO system is a little out of your range at the moment (CNS 17 is recommended to be run with RO filtered, or very clean, water) a simple brita faucet filter might be the difference maker here.
Ultimately, the high levels of Calcium in your system will create a cation issue for you to watch for.
My tap water starting EC is between 0.08 and 0.12 (measured as 80 uS/cm to 120 uS/cm), and this should be considered very clean tap water. If your own tap water starts higher than 0.5ec then you are definitely going to have an issue in coco. I actually am a firm believer in clean, pure, untreated water. Do some research into what is in municipal tap water systems that use recycled waste systems (most cities with pop. >20,000 south of Sacramento/Denver/DC).
If your water is over 0.25ec to start my suggestion is to take a look at your water report to see where it's all coming from. Could be a lot of sodium in there and/or bicarbonates which will force competition for uptake with other elements and could cause precipitation of salts down the road.
To combat the Calcium you're going to need to use a touch extra Potassium and Magnesium.
Dyna-Gro Pro-Tekt would be a good choice to balance your pH with as a pH up replacement as well as provide potassium and silicon for the cation balance. An extra 0.05ec (50uS/cm) of Epsom Salt would be a good addition, but Flora Nectar, Sweet, or
Bud Candy might be acceptable Magnesium supplements as well. I've tried
Humboldt Nutrients SeaMag and Organicare Huvega with no success, steer clear of those. GH Liquid KoolBloom 0-10-10 has a little extra Mag in it also, so that would be a cheap bloom supplement because you'd only need 1-3ml at most per gallon to balance the system out.
As always, watch your EC!
When you start adding new stuff that doesn't mean you begin using a higher EC. Use less of other stuff so you can begin supplementing the system slightly.
I think I said it above, but IMO 5 gallons of coco is WAY too much coco for an indoor grow unless you're growing a massive tree with a couple kw of lighting. Coco manages to support much larger plants gallon for gallon compared to soil, and in this regard is VERY different than standard organics. More bang for your buck with coco (even when amended organically) when it comes to space.
2 gallons of coco is a perfect amount to grow what I consider to be small/medium sized indoor plants. Here's a picture of my own garden I took yesterday. These plants are in ~2 gallons of coco in 3 gallon SmartPots.
So, you can see you don't need a lot of coco for big nugs.
There are many problems I saw when using high volumes of coco fiber in large containers. Compaction, even saturation, and reduced aeration were only the beginning. The big problem I saw was the result of feeding the entire 5 gallons of media when there was only 1 gallons worth of roots. Think of coco of containing a "piggy bank" of elements. A 5 gallon Piggy Bank holds more than a 2 gallon piggy bank, it's kinda simple, but for whatever reason it just didn't click for me until it was too late.
As you feed all that rootless coco it adds more and more elements. They aren't used, they just keep building up in the spongy coco, especially if the runoff levels aren't very high.
When the roots finally do stretch out into these areas they hit super saturated pockets and as a result the plant starts acting really funked. Then you're stuck running copious amounts of clean water through the coco to try and balance it back out and rather than taking off the plant has to sit water logged for a few days. The net loss is like a weeks worth of growth... Sucks...
At this point I think you just gotta work with what you got and try to keep them happy. Focus 100% of your energy on the root zone and the media, don't even try to play the reactive game of reading the plant.
:anim_09:
Focus on what you can control, ensure the environment is dialed temp/RH wise, the ventilation and air circulation is sufficient to bring in fresh air, and that the solution you are using is of an appropriate EC and pH. Then you just gotta put yourself in your "Roots' Shoes" so to speak. Try to imagine what it'd be like to be buried in the center of the media, and when to water, and to keep that runoff EC within reason.
If you can do these 5 or 6 pretty essential things the plants will respond slowly at first but exponentially as the root system rebounds. This will push healthy growth up top (the biomass you CAN see) and this then helps push healthy growth down below (the biomass you CANNOT see). When the two systems are working together then you're really growing :harvest:
If you cannot get the hard water thing worked out with the CNS 17... You are going to have problems no matter what you do. Dial what you can dial and make sure you address the Calcium/Cation issue you'll have running those nutrients with hard water.
Good luck!