What do the bottom of these cups look like? Do the roots stick out?
How do these cups get there nutrients? Where does the nutrient flow to?

"What do the bottom of these cups look like?"

They are perferated with holes around the sides and bottom. NFT

"Do the roots stick out?"

The roots sit on the bottom of the cup and the nutrient solution flows through the holes into the cup.
diameter holes are about 3 to 5 millimeters wide.

"Where does the nutrient flow to?"

It is pumped from a 5 gal. bucket and piped back into the bucket from the opposite end. It circulates very well.

Hmmm,
I think your roots are rotting. They should stick out of the holes touching the nutrient flow. Lift some plants out and examen the roots. They should be pure white.
If they are OK then put the plant back and fill the cups with perlite (or wood chips) Lift the cups up so that the bottom just touches the nutrient flow by now more than a few mm. When the roots come out, lift the cups up more.

Hi Kswizz,

Don't give up hope yet. I suspect you have nearly solved this puzzle due to your methodical approach.

Let's try to work through this step by step and hopefully isolate some possible suspects.

1. Given the 4x repeated results with varying brands of nutrients we can safely assume that a bad batch of nutrients that slipped out of the factory isn't to blame, lets look elsewhere.

2. It seems you have been keeping the pH of the reservoir in check so lockout due to extreme pH is unlikely.

3. No noticeable burning of the leaves. You have been carefully following the directions prescribed by the nutrients and keeping the EC within range so this together would indicate the plants aren't dying from excessive nutrients.

4. Your environment sounds good as far as ventilation and temperature go.

Is the humidity of the room reasonable? Something around 50 to 60%? I'm going to assume yes, but an extreme in either direction could be a contributing factor.

5. Young plants would be extremely challenged to deplete a five gallon reservoir of it's nutrients in less than two weeks. It's safe to say they aren't running out one or more nutrients and then dying from a deficiency.

I concur with Greystroke's suspicion that the roots could be rotting. If they are brown, slimy or undeveloped instead of being white and fuzzy as Greystroke described it's likely the problem can be isolated to the root system and now we just need to determine the cause.

Assuming the roots are rotting away:

A likely culprit is that the roots aren't getting enough oxygen. Plants absorb oxygen through their roots and water with low levels of dissolved oxygen promote certain harmful bacteria.

If the reservoir is much over 75 degrees F this could be be contributing to the problem. If the water gets too warm it can't retain oxygen. Having a small air pump that can bubble the water while it's inside the reservoir, preferably with an air stone, can help add oxygen to the water.

Are you starting the seeds in soil that's mixed with coco-coir or using straight coco-coir? Adding soil to a hydroponic system can introduce problem causing bacteria. If you are using strictly coco coir then you should be fine.

When the system is running how deep is the water that's running through those plastic channels? NFT systems are usually a thin film of water that runs 24/7. I'm not sure exactly how your system works based on that video. If the channels fills with a couple inches of water and then drains that works too, just shoot for 15 minutes on once every 2.5 to 3 hours. It may help as Greystroke mentioned to fill the cups with some pearlite and get the roots further out of the water.

Couple other things to think about.

Did you clean the system thoroughly between the 1st 2nd and 3rd and 4th try? It's possible that some bacteria or mold that killed your first crop has been living in the system ever since. A good cleaning with hydrogen peroxide or bleach followed by a rinse should eliminate this possibility for the subsequent try.

Also in the unlikely event you have some kind of mold living in the water pipes of your house you could buy RO water from the store and use that following a thorough cleaning of that system using a biocide like bleach. RO water costs around $0.65 per gallon here and for a few dollars you could get enough sterile water to last through the first few weeks. At that point you should be able to see signs as to whether or not history would repeat itself.

Good luck!

- Bilberrybrian

I throughly cleaned it out with bleach and rinse with fresh water. The first 2 times I used soil, the second 2 was coco coir.
So it sounds as you are assuming it maybe an oxygen,medium or heat problem?

Thanks greystoke and bilberrybrian, I'll see about purchasing a bubbler and changing back to perlite. I was using perlite earlier when I first started but it was very floaty and pieces kept clogging my drain but since I've re-modified the reservoir I shouldn't have a problem with that.

How do you germinate the seeds? In the grow rocks? If you do, start using rock wool that holds air/water in better proportion; after the seeds have 2 sets of true leaves, put the rockwool plug in the grew rocks.

I will guess that you aren't aerating the roots/nutrient and have pythium (root rot) as a result. Good nutrient aeration depends upon the solution temp being below 75F; see chart below.