As often as every day, although this typically isn't necessary. Prime® is safe to overdose by up to 5x the recommended amount in any given 48 hour period in most systems (See FAQ: Can I overdose Prime®? for exceptions). That means it is typically fine to dose Prime® every day, or even two times per day in most systems. Prime® breaks down completely after 48 hours, so there is no risk of a buildup of this product in the aquarium system over time.
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