I find that anything that ends up with stray organic goo in it, be it coral dip, fish transport water, or worst yet the water in the bottom of my bandsaw, tends to be full of waste which eventually turns to ammonia. It also breeds all sorts of unpleasant bacteria, which being non moving water tends to be the smelly, slimy, anaerobic variety. As a result it gets dumped. This is before accounting for any toxins that pests may give off when they die. I agree with dumping it out.