Can you get pics up?
Also, and this is important and I'm sure you know this- do you have any idea where the high ppm of your tap water is coming from? You're TOTALLY correct to dilute it down with RO but I'm wondering if the tap water has high mineral content, or if the ppm is from another source. The fact that it comes in at pH 6 worries me, because a lot of CaCO3 would actually have it coming in at 7.0 or even higher. I ask because you could be seeign calcium problems.
If you already run an RO unit, I'd strongly recommend skipping the tap water since it is a big unknown, and making water that is RO+ calmag up to 250 ppm, and then adding your canna coco ferts JUST before your feedings, and give them at ~6.4. WEAK, because your plants are young.
Also very important: How much B1 and Superthrive are you using? That stuff can put your plants in a damn coma. Yes I have killed my fair share, and then some, of seedlings with ST alone; you add ANOTHER hormone into the mix and you get trouble!

Post pics
Lose ST and B1
Run RO+calmag to 250 ppm in place of tap water
Give light feedings appropriate to life stage- 1/4 strength nutes when the plants have 4-5 nodes, working up gradually.

Good luck