When mixing nutes calculate the volume of actual water the system holds. Shut the circulating pump off. Add nutrients in the proper order to the test and mix then ph down to desired ph.
Ppm will be a little higher then intended until it mixes with the rest of the system. This prevents exposing the plants to concentrated nutes, ph adjusters and large swings of ph, salinity etc that can shock them or damage them.
How many sites are you growing in?
Use a pump that is about 10x the gph of the volume of the sites combined. Eg 4 5gal sites. 20gal x 10x change over per hour would be 200gph. Beyond 4 sites you will want to increase the flow (gph).
Suggest atleast 2" return lines.
You want an air pump that match the total volume required by the air stones. So say 6 4x2 cylindrical air stone run about 5-8lpm each so 6 would be around 40lpm middle ground. So an air pump of 600 to 700 goh would be perfect.
Something like this.
Eco Plus 728450 Eco Air1 Commercial Air Pump 1-18 Watt Single Outlet, 6 Valve Manifold for Aquarium, Fish Tank, Fountain, Pond & Hydroponics, 793 GPH, Silver https://www.amazon.com/dp/B002JLJC0W/ref=cm_sw_r_awdo_navT_g_M6XGJ3YE00C2W246QVKM