change in diet

We have found a local shop that makes up it's own dry food. We have been looking for a cheaper food to try on our six month old vizsla. At the moment he has two 200g meals a day of pro-plan puppy food. I've just been subsituting 50g of pro-plan with the new food. It's only been a few days but he is pooing for england! He's always been firm but now with only 50g added his poos are quite runny and lots more of it. Is this just due to the change or is it not doing him any good? Don't want to feed him loads of food and it not doing him any good. How long do I leave it and how much and when do I increase the new food? Was thinking it should take a couple of weeks but not if it's all just gonna go straight through him. He hasn't been sick at all which he does tend to do with new things that don't agree with his sensetive tummy.

I would stick to the well proven Pro Plan. What resources does you local shop have? Are they following a recipe that ''somebody says should work''? How long have any dogs been eating this food? What quality control do them use?

Pet generally shops don't make their own food (whatever they may claim) but buy it in from the two or three big manufacturers who make food for companies like Burns, Arden Grange etc. [link removed]They use the same ingredients, and you need to compare ingredients/analysis before substituting one for another, but they can offer good value.

However, if you change and get upset tum, then it could be something up with the food (rancid fat or something)