I know they get a bad rap but I've got a series of 50 gal Plastimo tanks in my boat. The origional tanks were bladders, on my FIRST 42. the boat was designed for all-out racing so the bladders made perfect sence, but this racer has turned cruiser and the tanks were replaced. I found the easiest and least expensive was the Plastimo Bladders. They are probably the easiest to install as you just roll them out and fill them up, and they settle into the boats contour.
I havent really worried about how clean they are as I'm one of the most picky people when it comes to water in my tanks.. Nothing goes into the tanks that hasnt been run through a series of filters including a carbon 5 micron.
I have to as my watermaker (spectra) has a clean water electronic flush where every 90 hours, it flushes the system with tank water. If I had any type of crud in the tanks, including bleach, or clorine, I'd destroy my system.
You might want to check them out..the price is right.......