I'm pretty sure your problem is not using conditioner. If your on city water you 100% either have chlorine or chloramine. This is why your fish act nuts during water changes
I currently have a 4 inch oscar with a 4 inch flagtail prochilodus in a 140 gal tank. They get along fine. I want to add another fish and was thinking of adding a 2 and a half inch flowerhorn. Anyone tried this with success?