I would be more concerned with the NPK @ 8-8-8 than the price of the stuff. I brew my own nutrient and keep it at around 1-2-1 +/-. Most hydroponic applications use a far-less potent combination of the big three nutrients. Then again, if you'd like your lettuce destroyed by tip burn way before any bugs show an interest, it might be just the ticket! Grizzman is right about tomatoes and raft systems, although "I" have successfully done it. The trick is 24/7 aeration -- large pump & air stone. Don't try the lettuce and tomatoes at the same time in the same pond though. Tomatoes are heavy feeders and lettuce is not.

Purple stems could be a potassium deficiency. The yellowing looks like an iron deficiency. Make sure your nutrient contains the deficient nutrients. Also, make sure the concentration is strong enough.

I used to recommend the GH 3 part flora series, too. But, I started using their dry Maxigro and Floramato this year. The price is way lower per gallon, you can buy a 1.5lb container for less than $15, and you don't have to measure with anything but a teaspoon. It also buffers 10 times better. I haven't added any ph up or down once to the tomato rez this year. Last year it was every other day sometimes. Of course, the nutrients aren't the only change I made. I also now have a single, much larger rez as opposed to individual 5 gallon buckets for tomatoes. Much better for maintaining ph and temps. FYI and maybe a non issue in this thread: I switched to Maxibloom for the peppers since the nitrogen seemed so high on Floramato. It's also a dry nutrient. It is much better. So, my 3 commercial favorites have just been narrowed down to GH's Maxigro (all vegetative growth), Maxibloom (peppers and eggplant flowering and fruiting), and Floramato (tomatoes fruiting). PH Meters: Get the cheap liquid drops. I found some disturbing issues with my Milwaukee pH meter. You don't have to worry about whether or not the drops are calibrated or if your battery is dying or your probe glass is crapping out or you're too close to fluorescent lighting. You don't have to buy storage, cleaning, and calibration solutions ($7 to $10 each). You don't have to worry about what happens if you drop it. I know I sound like a General Hydroponics employee, but I'm not associated at all aside from satisfied customer. Anyway, they make drops that are about $7. They last a really long time. At least one season. More if you have only a couple reservoirs to tend to. Here is a link that might be useful: PH Drops at bottom of the page at this link

Here is a picture of some of the die-off. What looks like a dead stem on the left hand side is actually a stick I've used for staking. On second thought, maybe that's not a good idea in the future as they are likely to carry diseases?