open neutral caused from changing water tank?
In short and to answer your immediate question-- yes, you most likely need an electrician to come perform troubleshooting. However, there seems to be some ambiguity here.

I'm going to make the assumption that "water tank" means 240V electric water 
heater? Did you install the new tank or was it installed by a professional? If all of this happened immediately after the installation, I suggest having your installer come back to verify the installation before hiring anyone else.

It sounds like your receptacle tester is not telling you the full truth. You said your receptacles work, but they physically cannot with an open neutral-- you would have an open circuit and be in the dark. However, you can have an open neutral and still have a live "hot" wire, so don't take anything for granted.

If the neutral is indeed open on all of your receptacles, that would most likely indicate a problem at your breaker panel. This would bring us back around full circle to calling an electrician.