I can help. You have two very big considerations here. First off, tomatoes and strawberries both want an EC of 2.0 (+-) while peppers and lettuce want something on the order of 1.2. They won't grow together using the same nutrient strength. Your lettuce will turn brown with all the nitrogen and your peppers won't produce.

While we're talking about lettuce; it doesn't like heat. It will "bolt" the first chance it gets. Bolting is the term given to a plant that is going to "seed" fast. You'll find that the plants get spindly and set flowers as well as turn very, very bitter in taste.

If you try to lessen the EC so the peppers and lettuce will grow, the other two will die. Grow these two groups of cultivars separately.

Next up: strawberries don't like their feet wet. That means, their root systems need to be almost dry (damp) to grow. Static culture would be a better method of growing the tomatoes and the strawberries.

If strawberries are growing suspended in water, you'll get all sorts of fancy-sounding scientific-named diseases, which all fall under the general heading of "crown rot". Some are fungi, others mold; bottom line, strawberries and tomatoes need full sun and well-drained growing medium.

Tomatoes "can" be grown in water and can even be grown using the "ramp" technique. If you choose to do so, make sure you oxygenate your nutrient water 24/7 or things get ugly!

If you need more help, private email me and I'll direct you.

The issue in DWC culture is heat of the reservoir and the DO (dissolved oxygen) Not many plants love their feet being wet. Strawberries may be more prone to disease if they are wet, but there is not many that thrive in saturated soil.

Roots require oxygen (hence perlite in soil grows) and if you submerge them in water.. they rely on the DO. If your plants are droopy check for root rot (roots should be white, unless using additives in your rez than will brown them) and should not be slimy in any way. If you don't have root rot then you might want to add more air stones, bigger air pump (more AIR!!) to your rez. Also DO decreases as water temp increases. You may need to chill your rez. Optimum temp for DO is ~70F.

You can buy a commercial chiller, place a fan blowing over the top of your rez, or do some DIY chiller (I've used an old water cooler internals).

Where is your rez? Above ground? Maybe you could bury it.. that's what i've done.

Hope this rambling helps.

--chris

The other thing you can do, which I forgot to mention, is using additives like hydroguard and SM-90 which are used to help control bacteria in the rez. They will allow you to run a rez at higher temps (though they don't help the DO issue)

These can help you limp along if you are having bacteria issues (root rot)

--chris

Ultimately the problem is you've got a combination of plants that aren't going to be happy drinking from the same reservoir.

Like freemansgreens said, what makes one crop happy will either kill or nearly kill one or more of the others, and vice versa. You might find a halfway point where everything just barely clings to life and looks sickly, but there's not much point in that. Decide what you like best and grow that. The other stuff either discard or move into a separate system.

In the paraphrased words of President Lincoln, You can please some of the plants all the time or all the plants some of the time, but you can't please all the plants all the time. At least not with the same nutrient solution.