Where does the aeration line to the retention tank enter the retention tank?
IMO it should be on top where the plug is now. And there should be a diffuser so the column of aerated water entering the tank is broken up in smaller streams to expose more surface area so the air can do a better job of oxidizing the iron etc..
The vent on the side of the ret. tank, does it actually allow air out of it? If not it's probably due to it being blocked. If it is blocked the water will be absorbing the air into it and that leads to less air to be used for oxidation. I'd want the vent on the top of the tank with a pipe going done to the water level; using a Honeywell float vent valve. That then sets the level of air and water in the tank always insuring proper aeration.
A retention tank hold water for a period of time for proper contact time between an oxidizer and the water. A vent tank does too but it is set up as I describe above; a retention tank is not set up that way and will not work as well as a aeration vent tank will.
The aerator injector should be 1" on 1" tubing. The oxidation process will load up the tubing past it with rust, and it can close off the ID of the tubing to like a 1/4", from the injector all the way to the ret. tank inlet. So that line should be very short and have the bare minimum of elbows in it.
Is teh backwash line from the filter 1/2 PVC (white) or 1/2" or 3/4" Cpvc (tanish/gray/off white)? If it is CPVC, it has to be 3/4", not 1/2" and I wouldn't have used it, I would have used 1/2" ID PE tubing with no elbows. That water flow is flow controlled and we don't want any further flow restriction because it will prevent proper backwash and that will kill the mineral in the filter real quick.