Hey kai. I wouldn't worry about it the pumps fine. That nut is a compression type fitting for certain hose. You don't even need that. I have an Eheim 1048 and had a leak right where your talking about. Go to Home depot, or the like, and ask for "teflon tape". If you don't know what it is it's a thin white (non sticky) tape to go on threads to stop water, gas from leaking through the threads. If you do know what it is ignore the last part.
