When I visited the famed underground cistern of Istanbul, which once provided a constant water supply for a massive palace, I noted that fish swam around in the water to “reduce bacteria”. Fish owners often use algae eating fish such as pleckos to keep tanks clean. However, the fish used in the cistern were massive fish that took the form of catfish or maybe even koi. After much wonderment I had the chance to look further into this method of decontamination myself. It turns out that even scientists debate the effectiveness of using fish to clean water. With a little reading I discovered that fish might consume algae in reservoirs in developing countries, but they certainly don’t purify water enough to drink it. Fish catalyze a reaction that reduces ammonia from rain water, making the water more suitable for life but not quite drinkable. Frankly, it sounds like an experiment I’d really love to try. You can read more about fish and their capabilities to clean water online. Here is the link to a discussion forum for this topic.