I've seen people who own many snakes do this possibly because they aren't changing the water very frequently. For the average snake owner with just a couple of snakes who should be changing the water every 1-3 days anyway I don't see a reason to add a penny (must be a copper one so only pennies older than a certain date work) especially if there are potentially unseen adverse effects. In any case, as long as your snake hasn't defecated in its water dish any water a snake would drink in the wild probably has more bacteria than water that comes from a household tap.
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