I have exact same dealio on my frag tank. My suggestion would be lower it all the way down and once you have the return pump going and get a feel on where you want your water level to be in the tank adjust accordingly. I have a pump that's a bit too powerful for my fragtank and with that level being raised all the way up it quickly filled my tank all the way to the edge so I had to pull the plug to make sure it wouldn't overflow.