That looks like a good system you have there. I have seen alot of posts on here about people getting in rough weather and their engines stall out. The general consensus is from the gunk in the tank that forms getting knocked loose and sucked up the dip tube.
Of course, this only happens in diesel tanks, since the microorganism that causes the problem only grows along the diesel-water interface. If you keep your fuel tank free of water, then you won't have a problem with sludge generally.
I cleaned the 18 gal. aluminum tank really well. I made an access port and had it tigged in the top. I didn't get it installed though before it came time to move my boat. I used a $20, 6 gal. plastic tank from a marine store. It has a little fuel gague and the fuel line screwed right in. It is in the aft locker. I thought I would need the 18 gal. tank, however I motored my boat for 10 hours straight and only used about 2 gal. of gas.
You mean diesel, right? Most more recent sailboats are diesel powered, not gasoline powered.

If your boat is powered by an Atomic 4, then you'll have a different set of worries.
I am considering not installing the 18 gal. aluminum because I am concerned that it will gunk up and then fail me when I need it most. I can easily look into the 6 gal. and if there is something growing in it I can throw it away and get a new one.
If you have the aluminum tank, not installing it is silly, provided you put a decent inspection hatch in the tank. If you have an inspection hatch, you can generally look in the tank and see if it is heading for a problem or not. You can also steam-clean the tank through the inspection hatch as necessary. BTW, you can't just throw away a plastic fuel tank if it has some fuel, sludge and water in it.. you have to dispose of the fuel properly.
Are there any problems associated with using plastic tanks long term?
Depends on the type of plastic.
If I don't have a high speed polishing system like Ken has, how often do I need to open up the tank and check it out?
Depends on how good the fuel you get is. If you buy at a place that has decent turnover, and fresh fuel, then it is just a matter of preventing water from getting in to the tank... as long as there is no water, you won't have the conditions for the sludge to grow. Using a good biocide in a clean tank as a prophylatic measure is good idea as well.
One other measure that can help is using a Baja type fuel filter funnel when taking on fuel at a location where the quality of the fuel is unknown or questionable.
BTW, using a biocide on a tank that is already heavily contaminated with sludge is just asking to replace fuel filters—since the sludge will die off and break apart and get sucked up into the fuel lines.
