I can HELP! I have BEEN here!
First, and I'm sure your vet knows this (he should), Giardia is notorious for hiding under scope - so it is very, very common for a vet to do multiple tests (and tests higher in specificity) when the first tests prove negative - when clinical indications *keep* proving giardia. My vet actually had to send Marcel's second or 3rd giardia test off-site to a different lab who could perform a higher-specificity test - and sure enough - there was one, teensy giardia cell hiding in there.
Above you said "high level of bad bacteria" - what do you mean? What kind? Another member recently had a very bad experience with clostridium - but that is usually (and quickly) life threatening - so it doesn't sound like your case. Tylosin - I'm not surprised to hear what's going on with it; Tylosin can be a lifesaver for some dogs; for others, a wreck. While it can help initially, it can later cause the problems it was initially HELPING, which is confounding. I stopped the Tylosin when Marcel was on it.
Now, is the diarrhea gone - but the tummy noises are crazy, correct?
If so, I would get him off the chicken/rice - and onto an even more hypoallergenic diet of whitefish (pollock, cod, sole, tilapia) and mashed white potatoes - and this is just a diet for short term, okay? What was he eating before this btw?
I don't want to write too much more until you come back - what is his weight/age? We can PM for amounts to feed etcetera.
Also, if he is a puppy - and IF this is giardia - there are other things to think about *now*. Giardia, in puppies, can actually begin to digest intestinal tissues - possibly causing long-term (and short term as you're seeing!) digestive issues. SO, you may want to give some nice supplements for about 6 months to support/heal any possible damage that's occurred - like probiotics, maybe digestive enzymes.
