I prefer to observe, as long he is eating there is no danger, and anytime you don't have to treat is best, but you will have time to act. Missing one meal is no big deal but if he goes off eating for a week, then treat immediately. If you see stringy poo, treat immediately. While cloudy eyes can be an indicator, not always, as they might just be shedding. But if not eating with cloudy eyes and lethargy, treat immediately. Also watch out for flukes, could be accompanied by swimming at the surface or hanging in the direct flow of a powerhead or high flow area, accompanied by going off feeding and maybe cloudy eyes. It all really does hinge on eating, just have gc or prazi, in the case of flukes, on hand and be prepared, and don't wait if you suspect issue. I have never seen any long term injuries so treating prophylactically wouldn't really be an issue, just make sure to dose to water volume and not tank size, the only issue I've ever seen was likely due to an overdose.