[erebus]

I put a poly in my 18 a few years ago.
Its really a much better alternative than burying an aluminum tank down in the bilge.
But...
Only if you can find one the right dimensions and capacity to satisfy your needs. Sometimes a custom aluminum is the only way you can go.

If you go poly keep in mind the tank will expand by about 3% once its filled with gas. Need to give it a little room when you install it.