I have Started a new Medical Grow due to previous complications. I will be growing Cataract Kush from Delisious seeds using a 10 Gal Dwc rubbermaid tote and will be vegging until the scrog screen fills up. I am currently using Humboldt Boldt County Master A & Master B. I have germinated the CK seeds and placed them inside root riot cubes. I Place the cubes about a quarter of a way down in he water to keep the seed from drowning on this past thursday. One seedling is about to appear while the second seedling has to redirect its positioning.
I think you have them in too much water. I looked up the root riot cubes and they seem similar to the rapid rooters that I use.
Either way, the instructions say. "Keep an eye on the moisture in the cubes, especially in warm conditions which will speed drying out. Ideally moisture should just come to the surface when squeezed. If not, gently water the cubes and then drain well. Be careful not to waterlog, this would encourage mould growth and suffocate the roots."
Water-logging may have been the problem with your PE plants. I'd keep them above the water level and drip from the top-down on a timer (about 1 min on / 29 off) with that setup. The air pump and stones should keep the bottom of the net pots wet and encourage roots to grow down. Once the roots hit the water (which should be below the net pots) you can stop the drip system. Also, make sure that your water temps are not too high.
Good luck...
Disclaimer: I still use HID and I'm NOT an LED expert. All of my LED knowledge is from other parties or research so I can't say from first hand experience one way or another. I also run a medical grow consulting business in SoCal.
1 Thing I do that really helps is, I will only put about an inch (or less) of clay in your pot, put in the cube then fill to top of cube with clay. This allows the roots to get into the res Quicker, which makes for a stronger faster growing plant, and also keeps light away from the cube, which will eventually lead to disease. So get them in deeper! You can continue to fill the pots with clay as they grow. Good Luck to ya! h
The pic is of a 2 week old using this method. Peace.
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