Once they poop yellow you don't have more than a few days usually, so it would be best to see a vet to get metronidazole *and* Baytril. They almost always have E. coli, and metronidazole won't treat that, you gotta have Baytril or something like Baytril. If you can't see a vet, go to petsmart or petco and get the fish antibiotics called "API General Cure". Check the back and make sure it has 250mg metronidazole and 75mg praziquantel. Then you will have to figure out how to give 1/2 a pack. Since it has praziquantel, I think you should give it just once day.