overkill or not

That's a canister filter, correct? Should be fine, but you may need to watch tank temperature since its pump is rather large.
 
Using a canister filter on a saltwater tank its always better to go for overkill. The bigger units give you more room for things like carbon and phosphate remover to really help keep the tank clean. Besides the heat issue Marc mentioned, you should be in good shape.
 
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