Different people have their own opinions, but I am a fan of pumping the water into a pvc spray manifold built over your root chamber. Drill holes with a 1/8" drill bit so that the water sprays down into itself. It should sound like a lot of peeing. As a jet of water hits a body of water, it will emit highly aerated bits of water as a spray. General Hydroponics' "Aeroflow" is a very successful design that capitalizes on that concept. I have also seen fish farmers utilize the same idea - you just squirt water down into more water.

I may be wrong, but it seems I've read that when growing lettuce in rafts, you don't need to aerate the water. you set some kind of "block" that keeps the raft from sinking as the water level lowers thus creating an air zone between the bottom of the raft and the top of the water. Ideally, the nutrient should all be consumed when the plants are ready to be harvested.
I know this answer is more or less a teaser, but hopefully you can do a bit of research and see if there is actually some validity to it.
I recommend starting here and searching this forum for raft and lettuce. I was originally made aware of this concept when a person posted a precise quantity of water needed per lettuce plant one time and the ensuing discussion. Of course it eventually degraded into a drivel flame war(at the time, it is just how this forum was), but there was a post or two that was useful.

Hello garret,

This is just based from my research, so its up to you what you wanna do with it.

Anyway, since you are building quite a system, one thing you can do is to aerate the solution as it goes into the drain pipe. With your system you would have 40 feet to do that, make it move like a stream or something. You can do this combined with using large air stones in the reservoir. Also it would be good for the plants if you can circulate the solution completely like atleast once every hourto bring oxygen rich solution to the plants. Even though it is dwc, plants still like moving water :-) just my two cents.

Also be careful of splashing water since i find the splashing tends to evaporate my solution faster.

I just got my greenhouse hydro machine up and running. It's recirculating deep water culture, just spraying the water back down onto itself. The spray line is attached to the underside of the lid, and I had problems with the lid sagging into the water in the middle. I filled one side of it with mini-lettuce, and the plant that was closest to the spray line that was functioning correctly grew much faster than the others that were all the way in the water. I suspected that would be the case. Lettuce may not need aeration, but it certainly likes it.

can you post some pics cole? :-) would be nice to see your setup

Sure. I am just getting started, so there's not much in it yet. The goal in this project is making a setup that will withstand the extreme temperature fluctuations of the greenhouse, especially in regard to the nutrient solution.

It's a lumber frame with metal corner brackets, 2x12s I think. It was placed over a gravel floor, so I put a smaller pea gravel down, then a layer of sand, then a pond liner over that. I had originally intended to grow mini-lettuce, which is why there are so many plant sites, but I am trying dwarf tomatoes, too.

The lid is two pieces of fiberglass bathroom paneling. Sandwiched between them is a piece of styrofoam, 3/4" I think. It's bolted together with machine screws. The spray line is 1/2" cpvc and attached to the underside of the lid. There are 3 lines running long-ways. The pump is a 580gph mag-drive pond pump. I think it all holds about 80 gallons of solution.

nice setup you have there. are those romaine?

Thanks. They mini-romaine varieties - Tennis Ball and Tom Thumb. The one in the pic is Tennis Ball. I like it better so far.

cool :) can you post a matured tom thumb and that romaine?