Hello ryeookin,
First I'm curious as to why you changed
nutrient manufactures? Did you think the Flora series
nutrients were not working? For plants that small your mixing them way to strong. Like I mentioned "seedlings that small don't need much nutrients at all". You were mixing the flora series fine (in tests 2 and 3) for a full strength nutrient solution. However seedlings that small don't need more than 25% to 33% of full strength. I will mix one gallon in a water bottle of full strength nutrient solution then, dilute it by adding 75% to 2/3's water in the system with the seedlings. The bonicare nutrients call for 7 mL for seedlings but I would probably go half that for seedlings that small. And your mixing them at 15 mL which is 1.5 times full strength.
That brings me to another point, I would never use so much water/nutrient solution for seedlings that small. It's a waste, they will never use it up. For just a couple starters like that I would probably just place them in a small Tupperware container, then just dip the cubes in a cup of the diluted nutrient solution once or twice a day. Just to keep them moist, but not soaked.
Reply to question answers
1. Having the bubbles splash the bottom of the cubes is fine, as long as the cubes aren't getting too wet.
2. First I'm concerned of what might be in your tap water. Second like I mentioned above your mixing them way to strong. I would recommend using RO water, or buy some bottled water for now to eliminate water quality as a problem. And yes mixing concentrated nutrients together will cause mineral elements to bond, that's why they are in separate containers to begin with.
3/4. If your room temp is between 72-74 degrees F, your water temp is likely fine (below 80F). However I am still concerned about the heat from the bulb being so close to the seedlings.
5. I asked about humidity because small seedlings like that should have high humidity levels (70% 80% etc.). They even usually benefit from a humidity dome (a clear plastic cover to hold in humidity). Also if the humidity level is low, that light bulb being so close just dries them out all the more.
6. The one that is dry is probably not getting enough water (drying out), and the other one is could be suffocating (not enough oxygen to the roots). It also could develop root rot or stem rot.
7. Seedlings that size don't need a lot of light, but they won't grow to full size like that.
cfl lighting should be fine for those type of plants, but you'll need to change your lighting setup.
8. A water culture system, is a water culture system. Eliminating the air
pump might be less expensive or easier, but it wont make your plants grow well. It's just doing what you can under the circumstances. I once used miracle grow instead of hydroponic nutrients because I ran out. Did the plants die right away? No. Were they still healthy a week later? No.
9. The root growth likely stalled because the seedlings aren't healthy anymore. Probably from multiple issues. Leaves, and/or roots drying out, roots suffocating, nutrient strength being 4 times stronger than they need. The one with the roots longer is likely due to a lack of moisture in the rockwool cube. The roots needed to stretch out to locate moisture.
10. Your pH is to high. That can also lead to nutrient deficiency, try for 6.0. For new growers I would recommend checking it every day, at least until you get used to how and why it can fluctuate.
11. Well I asked this question mostly to try and get you to think about the water quality your using. I'm surprised you went to the effort to find your local water quality report. Usually people just take a EC or PPM reading. But they don't know what those numbers represent. That was the point I wanted to make. Anyhow as I recommended earlier, I would use RO or bottled water for now to eliminate the question of water quality issues.