The last thing you really want to do is tie a furnace cold air return into a fish room, evaporated salt water will eat up ducting and furnace components quite quickly. You need a separate exhaust fan. Unless you're going to seal everything with lids (sump and tank) you're building a classic mold closet, I've seen at least a dozen of times in my days.
I'd stick with the melamine, it's cheap enough to replace down the road if need be and if you do a good job on the edge band it will be pretty well sealed.
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