Your question is a good one. It is certainly true that in an experiment with Planaria you would want to control what is in the water. And pond water could have particles in it that they could feed on. On the other hand, tap water could have chlorine in it that could kill the Planaria (It is usually added to the municipal water supply to kill disease microorganisms.) You could do one of two things:
1) Remove the particulates from the pond water by filtering it. Just pass it through a coffee filter or filter paper made for chemistry labs.
2) Remove the chlorine from the tap water by filtering it. Pass it through one of those filtering devices available in London Drugs or department stores for purifying tap water for drinking (Brita is one brand name).
Now you will have some good water in which to feed your planaria.
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