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Sorry, I just noticed in your OP that you were already giving him garlic with metro. Anyway agreed with Coryloach I personally also wouldn't give him that medicine for dogs, even if the ingredients were safe it'd be so easy to muck up the dosage. Is your fish still not eating?I'm not sure if your FH is completely past the stage of accepting food, but if you do try to treat any food with meds such as metro. etc. I'd suggest to mix in a bit of garlic extract (e.g. Seachem Garlic Guard), which will increase the chance your fish will take and keep the food down, but again I'm not sure if it's beyond that and not even nibbling on food. Also although it's not scientifically proven, there are many who believe that garlic helps to kill some pathogens, though I'm not sure if this extends to worms and protozoa. Good luck and hope he pulls through.
Yes indeed, Flowerhorn are quite prone to intestinal disorders -- I've seen that and other videos by that YouTuber and imho while the temp change during his water change might have weakened the fish to the degree that it enabled HITH to take over, imho he also went overboard on the crazy variety of foods he was trying to feed his fish. All a Flowerhorn needs is a good quality pellet or two and maybe only very occasional treat of frozen foods (and arguably not freeze dried krill, which some say can scratch their stomach lining, but personally not sure on that point).Sorry about the OP's problems with the flowerhorn. I don't own one. But I am wondering if these hybrid type fish have a propensity for intestinal disorders.
I was looking at this video last weekend and the owner was also struggling with what he believed to be HITH.