I suggest you see a vet just in case it is not parasitic or other reason, and I strongly recommend home made food. No one can tell me that fresh horse meat, organs, bones and veggies are not balanced, but dehydrated horse meat (hopefully) and starch are.
It depends on you, how dedicated you are on preparing good, balanced home made meals.
My B eats home made food. Meat, organs, bones I give raw, and I cook veggies and anchvies(or sardines). Once a year he gets probiotics and minerals (usually in April). Since feeding home made, he doesn’t react to grass (he used to have rash), no anal gland issues, no urinary infections, his coat is shiny, blood work is great.
That is my experience.
Buy books about home diets, do your research and remember not to overdo with the proteins.
I am copying a recipe that I recieved from my Vet. It indicates that you should maintain this diet while providing distilled water. Then gradually expose the dog to foods one at a time (beginning with tap water) to discover what the dog can eat (without a reaction). I found it interesting that tap water could be the culprit, but water does vary from one location to another. Anyway, this may be worth a try...
Canine Hypoallergenic Diet (from Hill's Pet Products):
1/4# (.11kg) lamb, diced
1C (150g) rice, cooked
1tsp (5ml) Oil (it lists vegetable, but others should work as well)
1 1/2tsp (7grams) dicalcium phosphate (aka one Tums)
Trim fat from lamb, cook thoroughly without seasoning. Add remaining ingredients and mix well. Keep covered in the refridgerator. Makes 2/3# (.30kg). plus a canine multivitamin/supplement
Protien ... 7%
Fat ... 10%
Carbohydrate ... 14%
Moisture ... 66%
Metabolizable Energy ... 795kcal/#
(I am not a scientist... but, it is my understanding that the average 20# dog requires ~400kcal/day, so I would think one recipe per day(?).)
10# ... 1/2# (.23kg)
20# ... 1# (.45kg)
40# ... 1 1/2# (.69kg)
I would recomend that you get a little notebook and begin making daily notes on what your dog eats and how it affects their activity level, alertness, etc. Any observations you can make. Also, don't expect to see instant changes in your dogs coat. It may take a little while before you see the situation improve. Keep that in mind, nothing you do is going to mean your dog will wake up tomorrow with a beautiful coat. it takes time.
Speaking of tap water, I only use filtered water at home for both their water dishes and water on their food. When traveling I use only bottled water. Depending on the water you use... if you look at the residue in the bowl, you would not be drinking that either... we have very hard water, reason for filtered. And when traveling, bottle is best so you don't have a change because elbrant is correct, water from one location to another varies.