It looks like your EC is too high as you said.

I have looked up the Dyna-Gro range, I assume you are using the Dyna-Gro Grow product and not any of the others? This is what you need for your lettuce, vegetative growth.

When you grow from seed or start to grow using small seedlings then the EC must be at a reduced rate. By feeding them the full 3 tsp you are giving the plants the maximum amount of grow product - this is going to be way to high for your small plant.

Now, Dyna-Gro (as far as I can see) don't give dosages for starting out, this is bad... You have no way of telling what you are putting into your tank, unless you buy an EC meter.

If you can get hold of a meter the battle is won, you will then be able to give the right amounts of nutrients to your plants.

Lettuce, generally, don't like high EC levels, they are a 'delicate' plant. When in full swing would I put no more than 1.5 into your iceberg.

In the interim, you are a little stuck. You could 'flush' the system through for a few days using pH water (adjusted to 6.0) to remove the excess nutrients. You can then reduce the amount from 3tsp to, say 1tsp, and see how this goes but, again, you can only guess your EC - which is impossible...

From what I can see, you are doing everything else perfectly. Sort yourself out with an EC meter and you will have all the tools to do hydro. really well.

There is one more alternative, purchase a better brand of nutes that tell you how much to add during the stages of growth e.g. General Hydroponics, general dose is 1 tsp going up to 3 tsp for the grow mixture.

I just purchased an EC meter (TRUNCHEON) off of Ebay today. So in the mean time I replaced my nutrients down to 1 tsp/gallon. I hope they start to rebound until the meter arrives. I'll update results. Thank you for your input.

I had a problem awhile ago, but mine was an anaerobic bacteria; I have a NFT system, too deep a water (didn't set my drain pipes deep enough), not enough oxygen, maybe light leaks in the net pots and possibility of too strong nutes... similar problems; brown roots and leaves wilting badly. My fix was; first flush was fresh water, secondly added H2O2 (hydrogen peroxide) 45ml in 30 gallons of 35% H2O2, the leaves perked up very quickly, I lowered my drain pipes, no problems since.

Here is a link that might be useful: My First Hydroponics System

Just an update (1 week later).
I changed the nutrients out to 1 tsp/gallon.
Total transformation and going green again with new white roots. Thank all of you who helped!
New photos below of the exact same plant and it's new roots. It has actually grown a little too despite the setback.

Here are some suggestions for you. When you change out your nutrients, remove the lid from your container (with it should be your plants), and place it on another container that has nothing in it, but maybe water if your going to be long in your process.

Clean your container with a wash cloth and soap and water, make sure there is not soap left as you rinse it very clean.

Add water to the level where you normally operate your container, drop in your air stones and then slowly add your nutrients until you get to about .5 EC.

When you get there check your PH. For lettuce, anywhere between 6-7 is okay. For me, I set the PH to 6.0, and then it should creep up to 7.0 over time. When it gets to 6.8 or so, then remove the lid and add PH down to get back to 6.0.

Many folks add the PH up or down or change the nutrients while the plants roots hang in the water. This can cause your plant to have issues.

Also, if you spill your nutrient solution on the leaves, they will also turn brown or yellow.

If your roots are white, they are getting enough air. If they are brown / rotting then they are not getting enough.

Make sure clean the media very well before using it, and if you are wise you will flush the media each time you change out your nutrient solution.