It certainly isn't ideal but 18C for a short period of time is no need to panic. If you have super delicate SPS it may be an issue but with most LPS and softies and hardy SPS and fish they can handle it for a short period of time.
In June my tank lost power for 24 hours and my tank got down to 17C. What's actually more of a concern in a power-out situation is the loss of oxygen and water circulation.
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