Yankee, what you have is just trapped air in the canister portion, and is common when you first start the sucker up. It goes away over time if you wanna wait it out (slowly escapes). There are ways to remedy that, such as: (1) start it up with as much water as you can, without making a mess when you reattach the top, and then once started, you can invert it to get the bubble out, or (2) mess around with the input/output valves.
The flow on the Eheim will be quite strong when you start out, but over time, it'll slow down naturally by itself (from filter material getting clogged to intake getting clogged, etc.). Lengthening the spray bar will help reduce the flow back into the tank (would be best to situate it in the back wall and face the output towards the front to reduce flow). You don't want to move it any further down the tank, since the water flow will just aid in uprooting your plants. Have it positioned a bit below the water surface (1-2") at a slight downward angle.