Hypochlorite (like they add for disinfection) will break down on standing, or faster with aeration. Chloramine (also added for disinfection) won't, which is why

Hypochlorite (like they add for disinfection) will break down on standing, or faster with aeration. Chloramine (also added for disinfection) won't, which is why aquarists use various chemical treatments to remove that. Neither is too important for plants, though. The usual problem for plants is chloride, which can't be removed except by processes like reverse osmosis or distillation. Most plants can easily survive chloride at typical concentrations in tap water; but if you don't water to runoff then that concentration may slowly increase in the root zone, eventually harming the plant. You typically want something around 10% runoff, though that will vary with your plant species, water, fertilizer, etc.