Affliction hitting only a couple plants in shared reservoir

Third grow, new to using a little more automated system. Several of the plants are wilting dramatically and quickly, white tips on leaves, but others are completely fine.

System:
Using the "ebb n grow" complete system from CAP, it's an ebb and flow hydroponic system. I use RO water, cyco platinum series nutes (following the advanced guide exactly), 25 gal in 50 gal reservoir for 12 plants in 2 gal pots. Growing medium is cyco expanded red clay for good drainage. Room stays about 75 degrees (as good as I can get it given the room and my limited hvac budget). Growing under a couple 11 band custom led fixtures a local guy made, these have proven to work well for my past grows, 18/6. Floods for 15 minutes 4 times a day evenly spaced (timer only goes down to 15 minutes). Halfway through week 3.

Affliction:
A couple of the plants looked like they were being overwatered, so I backed off to 3 times a day and tried removing the afflicted ones from the reservoir entirely, no help. They continued to wilt like you would expect underwatered plants to do over the next week, finally stopping growth entirely. I expected root rot (I can not see the root system because they grow entirely in the medium so I couldn't check) but when I pulled them out today, they look relatively healthy, though stunted (attached). Plenty of new root growth and some older stuff slightly stained from the chemicals, but not slimy, no bad smell or particularly dark/clumpy. Reservoir got to 82 degrees F before I knew root rot was an issue, I've since got it down to 73 degrees. One of the other seemingly healthy ones a day ago had nearly completely white and dead leaves today. All of this right next to completely healthy (almost ahead of where I would expect them to be) plants sharing a reservoir.

I'm leaning toward some kind of disease at this point? I noticed a couple black goo-like spots on the Grodan starter cubes that I used, I cut them out but there could be more where I couldn't access. Any help diagnosing this problem would be greatly appreciated, it's proved itself capable of spreading and killing plants in a matter of 12 hours and I'm afraid I will quickly lose my entire crop. Thanks in advance to anyone experienced and willing to help.

 

I wanted to follow up on this in case anyone runs into similar issues. The black gunk ended up being algae. Letting my reservoir get too warm temporarily provided a great breeding ground for them. They provided too much competition for oxygen for the root system of the plants, which produces the same negative effects you'll see from over watering because it is the same issue with a different cause. I think the algae only spread to a couple of the buckets at this point, which led to the variation plant to plant.

I am removing my plants and "nuking" the system with high concentration H2O2 tonight, then draining and starting my system fresh on their typical schedule. I believe keeping the reservoir temps at the right level will keep this from happening again, I'll follow up if this doesn't work.

 

The algae was pretty bad. My more developed plants just had better root systems so they made it through better. Added an air stone, controlling the temperatures, and everything is significantly better 18 hours later. Also added my tent material to cover my growing medium so no light gets to the medium, which should also help reduce growth of unfavorables. Thanks for the reply!

Sent from my VS990 using Grasscity Forum mobile app