Why do you give them straight water every 4th day Nuk? A habit from growing in soil? Pretty much everyone on here says no straight water in coco until final flush. I'm running 6/9 at half strength on clones in hempy cups right now.
A friend turned me on to 1/2 strength nutrient feedings between the full strength ones and I have had good success alternating full strength and half strength solutions.
I would do this by mixing up 15 gallons of nutrients, using 10 today, then adding 5 gallons of water back to the remaining 5 gallons of solution for tomorrow. It was easy to manage and the reservoir doesn't get funky in a day (usually).
I think you always want something in your water, even tap water is okay, just so long as it isn't RO filtered and <50ppm. I like going with a 1/2 strength solution especially when I'm looking to get a little extra runoff that day because the EC has been climbing a bit.