I have a 5 1/2 month old English mastiff puppy. For the past 2 weeks he's had very loose stools. He's been tested for worms and giardia which came back negative. The vet said he may not be getting enough fiber in his diet. I have him on Premium Edge large breed puppy food and the vet gave me Hill's W/D for weigh gain control (adult food). I don't think I can afford both brands of food. Do I continue him on the puppy food and find a fiber supplement or should I switch him to adult food higher in fiber? He currently weights 83lbs. Is he growing too fast. The vet didn't give me any solid answers to go on. I reallly am lost at this point.