Brass stands up fine in salt water. It goes without saying that it should not be immersed in water or let water drip from the metal into the tank. The pendants themselves are painted or enamaled to protect from corrosion. I'm not sure I understand what you are going to use the strapping for. (Actually, come to think of it, Stainless steel would probably work fine for this application as well. I use SS screws to hold my reflectors in place and SS nuts and bolts to mount my fans)
120VAC fans are available from Radio Shack, Princes Auto and electronic supply stores. The most common size is 120mm or about 4.75 in. ( some people would call this a 4" fan because the blades are 4" across) Heat shrink tubing on the connections or wire nuts (marrets) filled with silicone sealer or vasaline will protect the wire.
For a 90gal tank, one fan on either end should be suffice.
Simply put, blowing gets the moving air where you want it the most and causes the most directional movement of air. Look at it this way; when you want to cool off your soup or your coffee what do you do? You blow on it! Fans can only move a certain amount of air at any given time so having one fan blow and another fan suck doesnt move twice the amount of air. Have all your fans blowing in and as long as you provide plenty of escape routes for the air (ie. open back, vent holes, cracks along the lid/doors etc) it will find its own way out. Like I said earlier; sucking moist, salty air thru an electrical component is a safety and reliability issue.
The main purpose for fans in the canopy is to keep your lights cool and to PREVENT the heat from the lights from heating the tank. A fan in the sump is a good idea to get rid of extra heat if you find your tank is still getting too warm. One word of caution on blowing air onto the lights; some metal halide bulbs (especially cheap ones) dislike having air blowing directly on the bulb. This is usually not a problem with pendants but may be a problem for others. Air deflectors or repositioning of the fan or bulb may be neccesary.
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