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Going instantly from darkness to bright aquarium lighting can create an abrupt environmental change for fish.
Aquarium guidance often recommends allowing ambient room light to provide a transition rather than suddenly illuminating a completely dark tank. Consistent day-night schedules can also be maintained using timers.
Different species have different lighting requirements, especially when live plants or corals are involved.
Lighting an aquarium isn't only about making the tank attractive to the person looking at it.
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