It's ok to give human probiotics. I gave them to punkin and others here have used human probiotics for their cats too. Start with a small amount and you can gradually increase it. Too much probiotic can also cause diarrhea. I'm wondering about you having leftover metronidazole - usually when it's prescribed you give the entire prescription, unless the vet directs you otherwise. Is there a reason you had some at home, ie, did you finish it whenever it was prescribed? We had very good luck with metronidazole.
I'm wondering about the stool you're describing though, that doesn't sound like "ordinary" diarrhea. Just fyi, fish and beef are 2 common food allergens for many cats.
It looks slimy and yellow with undigested pieces of food in the midst
The cat's gut produces mucous (slime) when it's being irritated. That helps protect the colon from whatever is in there that's irritating it.
So whether you've got a food intolerance, infection of some sort, or EPI it's hard to say. Usually cats with EPI have stools that people describe as gray, large and bulky. I really don't know much about that, however, I'm just repeating what I've seen people say, so take it for what it's worth.
If he's had part of his bowel removed, that alone can cause diarrhea. Does he ever have normal stools? The pumpkin that Gill mentioned is used for the fiber to help regulate things. You can buy canned pumpkin (nothing added) and give a small amount, like 1/2 -1 teaspoon daily added to his food. You can also buy baby food squash (no sugar added, no onions, no garlic, no spices) and see if that helps.
There is are 2 good websites on feeding cats appropriately - from the "
Where Can I Find?" thread:
They may have suggestions on dealing with diarrhea as well.