DWC water level

Then I must not complete the water level in middle of week and just do a water change one time by week?

The best way with DWC is to do weekly water changes with fresh nutes this will prevent a lot of issues. Keep the fresh water in a bucket over night so the chlorine and any other chems will evaporate before adding to reservoir and keep the ph around 6.3-6.5 if it drops under 6.0 your plant will start to show signs the look like nute deficiencies, ph can change very quickly with DWC so it's best to check it daily. Another thing is the water temperature if you can keep it around 20 degrees cel so you might have to get a water chiller and keep the air pump in a cool place that has fresh air because it will also heat up the water.

I start out 1/4 dose of the nutrient solution chart for my nutrients for the first week, then up it to half dose for the second week. Then you can feed nutrients or strait water depending on how the plants are reacting to the current nutrient/water situation. Ph is critical to stay between 5.8-6.2 and water temps between 18c-20c are ideal. If you don’t have a RDWC with a reservoir you can use a fish or pond pump to pump from a spare bucket with your new water/nutrients to your DWC bucket if you want to use a screen later on. If you are doing multiple plants you can setup an RDWC with a reservoir and makes it much easier in my opinion and gives you more options for scrog ect. Hope this help! Cheers💨