6.8 will work, but thats as high as you want to go. The 5.5 range is preferred but takes work to maintain. You could drop your PH every other feeding, the lower PH is to allow certain micro nutrients to be taken in and used more efficiently. You can have a lock in your coco but it will start at your reservoir. Grab your nutes and head to google, different forms of minerals can bind to each other and become almost unusable to your plants. That can be from your PH, the types of nutes and forms in which they are delivered and temperature can be a problem as well. Mix in some hot/warm water to your nutes and some minor PH adjusting and you should be fine. I'd mix in some red lava rock with your coco, I'm doing 50/50 mix on this next go. The lava is already acidic and just like the coco it acts as PH buffer, so you might have 6.8 in the feed but it could be around 6 in the plant substrate. Coco just plain wins.