Thank-you for rescuing your 8 year old girl.
Canned food is too expensive to be feeding such a large breed. I would suggest you go to a good kibble that is higher in calories, and only feed a tablespoon of canned food....or go with some fresh food toppers to add to the kibble. Half a can of sardines, left over lean meats, yogurt, cottage cheese, left over steamed veggies, a raw egg or a lightly scrambled egg.
Good quality foods often recommended here are Innova, EVO, California Natural, Canidae,Orijen,etc. These foods are high in quality. I tried Merrick a couple of years ago...and was not all that impressed(kibble)...it's not high in calories and the dried veggies went right through my dogs.
Gina