You're somewhat right to be worried, an overdose can asphyxiate fish by too thickly coating the gills. It takes about an 8 to 10 times overdose though.
Still, it's one reason I prefer Prime. Detoxifies chlorine, chloramine and ammonia. No slime coat additives or alleged anything else. Doesn't take a bunch to do it either. I like stuff that actually does what it claims without ill effects.
Good to see you posting!