It may cause some what of a shock, however it's just like doing a larger water change. Shouldn't have too much effect on things, if you SPS corals you may see a bit more of an effect but nothing deadly.
One thing to keep in mind is that you'll be increasing your net volume meaning you're porbably short on rock and filtration equipment, so try not to keep this going for too long, otherwise you may run into problems.
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