I'm hoping rust (at least enough rust to cause a screw to fail) won't be an issue in the foreseeable future. (Maybe by then I can afford to buy those glass 180 or 240 gallon tanks

) The ends of the braces are still glued (reglued) to the insides of the tank so no water contacts the screw from inside the tank. I know that spills and splashes will happen to get saltwater on the screws from the outside and humidity will be an issue, but remember the Delorean (sp?). I suppose I could cover the screwheads and washers outside the tank with silicone for longer life too.
At our club meeting yesterday, tschopp, David and I discussed using nylon screws but I chose SS instead because I pictured having to tap the holes to use nylon screws and the SS can cut it's own threads as they get screwed in. Plus I figured SS is stronger than nylon.
scubadude- Can you explain a little more what you mean by a screw with a nut, I'm having trouble picturing it. Do you mean a short piece of threaded nylon (like a bolt) with a nut on the outside of the tank? (I'm having trouble seeing what the difference would be between; 1) a screw with a washer and 2) a bolt (or short piece of rod) with a washer???).... OR do you mean a longer threaded nylon rod to span the entire width of the tank with a nut on both ends? (then the threads are only holding 1/4" inch or so where it passes through the nut.) BTW, the screws are directly into the cross braces right now, but they seem solid. They are 1 1/2" screws so it is basically a screw with an 1 1/2" long pvc nut. I have been pushing around on the tank throughout the day today to make the load more dynamic and stress it more than a tank of water just sitting there.
marc- I don't take your input the wrong way. I know you have a solid reputation for good, honest, helpful information all around RC so I know what you say is a sincere attempt to help. As always, I welcome your input. There are allot of good ideas coming into this thread and even I don't know what step I'll take next so the more creative juices flowing, the better.
Maybe I can ask my dad to take some of the pvc square stock with a screw in it to his lab and see how much stress it will take to cause it to pull out. I'm thinking the load on each screw is around...(8.3 pounds per gallon X 210 gallons=1743 pounds / 10 screws = 174.3 pounds per screw.) Plus the load should be spread a little more from the 1 1/2" washer. I can't see 174 pounds pulling out an 1 1/2" screw, but I don't know if that is the correct way to figure the load or not, but I'm betting someone does.
Treeman- How long were your tanks set up before they warped?