Well, I was going to use it on a 50 gallon, so it will be fine for 70 gallons, though you will not see any dead spots! As I understand it, it has two outlets, so you'll need to find a way to spread the water evenly throughout the tank, but as it has a heater built in, it's one less thing to have in the tank. You will be filtering the water much more than some do, and less than others, but, and I know I'm not the seller, and hope Ren&Stumpy doesn't mind me dipping in, if you can afford it, Eheim is an extremely reliable product, and there are plenty of aquarists out there who have had them for years with little or no problems, me included.
Martin