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Hi. My name is***** Thank you for your question. Congratulations on the new addition to the house. I'm going to say that eating too fast shouldn't cause diarrhea. Eating too fast can distend the stomach and cause vomiting in my opinion. As far as the diarrhea that this little one keeps having, it may be his intestinal tract needs a little more time to get used to the diet that you're trying to feed. When making the transition to a new food, I like to have 1/3 new mixed with 2/3 old for 3 days, then 1/2 and 1/2 for 3 days, then 2/3 new to 1/3 old for 3 days, and then lastly all new. You could in this situation mix a little of the new food in with the ground beef and rice and progressively go to more and more of the diet. This can help his intestinal tract get used to the food. As far as probiotics go, this may help here. Here is a link on Purina's Fortiflora which a lot of clinics use.
It wouldn't hurt to screen another stool sample at his next vaccine visit to make sure there are still no underlying parasite issue. Sometimes the giardia can be tough to diagnose.
The water quality coming out of one cities faucets compared to another shouldn't make a bit of difference. I would place no suspicion on that water source (unless he was drinking from a lot of puddles, creeks, ponds now). The giardia can be a tough one to diagnose. Just not finding it on a single stool evaluation doesn't mean it isn't there.