Hi Christian,
Not Susan here, but I do have prior experience with this behavior also. It does sound like the food is giving your puppy acid reflux, and apparently the vets do also, since they are telling you to change food and use Pepcid. You said that it increased after changing from puppy to adult so it would appear there is more of what is causing the problem in the adult food. I do not think the problem is adult food, but the ingredients in Merrick. My girl that is grain intolerant was throwing up in the mornings when I fed her Blue Buffalo grain free, but she quit immediately when I changed her food to Natural Balance limited ingredient. I now feed her Wellness limited ingredient and Natural Balance plus she gets a topper on it.
I agree with the vets that you need to try changing foods first because that will give the vets a wealth of information. If changing food makes it worse or improves then you know the cause is food, and not something else. This is easier and cheaper to do than running tests.
Also, the last thing you should do is add more water to the food at this point. Additional water will make them throw up more when they have acid reflux due to food. As Susan will tell you, when the acid reflux is really bad, the dog will not want to drink at all because it makes them sicker.
I would recommend the NB Potato and Duck or Sweet Potato and Fish since they are both very limited ingredients and it is best to start simple when trying to determine if there is a food issue.
