[GpsFrontier]

Hello issaconan,

brand of good water pump + technical sheet

As far as brands of water pumps, it doesn't really mater. Any brand of submersible pond or fountain pump should be just fine. Just make sure it's big/strong enough for your needs. And I know your next question, that's why I wrote this article: What size pump should I get? As for "technical sheet" each pump should have the specifications right on the box.

nutrient needed and how to use it (doses + how many time)

Make sure their designed for hydroponic plants. For growing lettuce you don't want them to flower or seed, so you want nutrients designed for vegetative growth. Their are tons of manufactures that make good hydroponic nutrients, but they don't need to be expensive to be good. I have started a list of cost effective nutrients, and add them to the list as I find them. Cost effective nutrients. Each manufacture is different, and will be able to provide their own instructions. Typically you want to mix them on the weak side, especially for lettuce. All you need to know from the nutrient manufacture is how to mix them for a full strength nutrient, then you can dilute or adjust the strength of it as needed from there.

change water tank or not? if yes when

Yes, you will want to change the water in the nutrient reservoir. When and how often varies and has many variables. And again I wrote this article: What size reservoir do I need? to help answer those questions.

oxygen pump is necessary?

No, Not for an NFT system. It won't hurt anything, but it wont really help anything either. First the roots hanging in the air inside each of the rows of the NFT channels will allow the roots to get plenty of oxygen. Second the constant falling water back to the reservoir will be it's own oxygen pump. With the falling water acting as the oxygen pump, the only reason the dissolved oxygen levels should fall low would be from high water temps. And if your water temps are to high, that's a much bigger problem.

setting timer?

What timer? A N.F.T. system runs 24/7 so it doesn't use a timer? Your growing outside so you don't need artificial lights that would be on a timer? Along with the other articles I already posted above, I wrote this tutorial/description on how N.F.T systems work as well. I also wrote this article: Tips for Growing Plants in Hydroponic Systems

Considering you have a goal of 100 total lettuce plants, this is how I would calculate harvesting and production for them. First most lettuce plants can go from seed to harvest in 6 weeks. But this may be different and even 8-9 weeks depending on the specific variety, so you'll want check seed to harvest times for your specific variety. For this example I will use the standard six week time frame.

Weekly harvest 100 divided by 6=16.6 plants able to be harvested per week.

Starting seedling transplants to rotate into the main system to replace harvested plants.

In order to transplant seedlings into the system to replace the harvested ones, you will need to make sure the roots are long enough to reach the bottom of the NFT channel and the flow of water. The easiest way to do this is to build a secondary small water culture system to propagate the seedlings. The seedlings can be packed close together because they wont be very big before their transplanted. They should be in this propagation system for about 3 weeks before their big enough to need to transplanted. So if your harvesting 16.6 plants per week, and the propagation system holds 3 weeks worth of seedlings that means the propagation system should be able to hold at least 50 seedlings (16.6x3=49.8).

Now with 3 weeks worth of seedlings in the propagation system (50 plants). This means four things.

1. If you only want 100 plants total, then you can downsize the main system to hold only 50 plants because the other 50 will be in the propagation system.

2. If you still build the main system to hold 100 plants, and you have 50 in the propagation system, you have a total of 150 plants. With the 6 week cycle you can harvest 25 each week.

3. Or you can increase the amount of seedlings to 99 so you can harvest 33 plants a week from the main system.

4. Or you can just leave the plants in the main system longer to get bigger before harvesting. But regardless you need to know the seed to harvest times for the variety of plant/lettuce your growing.

Little tip
Once you run the numbers and know how many plants you will be harvesting each week. I would design the system so I can harvest the entire row of plants so the NFT channel can be cleaned properly between harvests. If you still have plants growing in it, you can't ever take it out and clean it. Then over time dead roots and plant material will build up in the system causing pathogens, and you will have to shut the whole thing down to clean and sanitize it. Then start all over.