a python vac is basically like a gravel vac only you can connect it to your faucet via a long hose. when you turn on the water, it creates a suction and you can begin gravel vaccing immediately. to add water to your tank, you flick a switch and it starts to add water from the faucet to your aquarium. you might think the fish will die from chlronie or heavy metal poisoning but that won't happen. before people start pumping water back in, they add enough water conditioner to treat the entire aquarium size. just guesstimate the faucet temperature to match your aquarium water as close as possible. and then your done, a lot easier (too bad i don't use a python yet...)