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In my opinion you can use bottled water. Put some dog food in and a little bit of dirt (say 1 tsp.) The soil introduces bacteria to the water and the dog food gives them something to feed on. All too soon you'll have some nice stagnant "pond" water. Rain isn't sterile either, so you only need to start with that bottled water. Using tap is ill advised since the are agents in tap water that purposely kill bacteria, even a brita won't remove those, so just grab a gallon of water.
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