You should leave a small amount of air in the tank. This way, the developer can move when you agitate; if the tank is full, there will be very little movement of the developer, which can lead to agitation related problems.(posted 8067 days ago)The easy way to do this is to tilt the tank when filling. When the developer is overflowing while tilted, stop pouring, then turn the tank upright. This will leave just enough air for agitation, without having to worry if the reel is totally covered.
David Carper ILFORD Technical Service