Aluminium is the preferred material for fuel tanks. I have cut and pasted a section by another Ericson owner from a previous discussion on tanks below.
Freyja E35 #241 1972
Here is the general consensus on tankage and materials. Also, interspesed with years of working on boats/yachts and seeing the results of what happens if it isn't done correctly.
For small craft:
Al. for fuel, S.S. for water, and plastic for waste. Why?
Gold plated tanks......think about it....non corrosive, looks good...... Just kidding. Epoxy coated waste and water tanks with straight steel or Al. fuel tanks.
Al. is fine for gas and diesel as long as it is of signficant wall thickness (>0.125"), well secured, baffled correctly, and not sitting in a puddle of water (or touching wet wood, etc). Robust, permantly installed fiberglass tanks are usually OK and Hatteras uses them in their boats but negates the ABYC principle of access and removability.
However, the trouble starts when we start screwing bronze and brass onto the Al. tank, bilge water constantly lapping underneath, etc.....so be mindful of the 'galvonic' effect. Also, the tank must be bonded to the fill to prevent static discharge and resultant "KABOOM" effect (gasoline especially!) and approved fuel hosing through out. As for plastic fuel tanks, well, first off, the USCG doesn't even recognize them as acceptable for storing gas inside a boat for passenger carrying boats, why should you? They crack, waffle, heat degredate, prone to vibration cracking and usually don't have baffles in them.
Think of a good several days at sea with a pounding head sea, the plastic is just not up for the job and may self destruct over a period of time. Maybe the USCG is onto something....... but their cost is tempting. S.S. fuel tanks are VERY pricey, save it for your water tank. Also, I don't think the ABYC reccomends S.S. tanks unless cylindrical and domed on the ends. Hmmmmmmmm.......
As for Water, S.S. is preferrred over Al. as it has less potential to react with cholrine in the city water we dump in our tanks. Here in South San Fran it's so bad, the water tastes like a swimming pool. Keep in mind plastic is permeable, and should AT LEAST be FDA certified for potable water. Regardless, there is not a plastic tank out there I have not tasted some plastic residue (and that doubles for the FDA approved potable water hoses). Also, see the 'self destructing' point made above.
As for waste tanks, well, our body fluids are pretty corrosive (urea, ammonia, bacteria, etc.) then mix it with sea water and it will have a pretty negative effect on all BUT plastic. So what to do? Well, THICK walled plastic tanks are about all that will stand up to the corrosive waste brew, but again, it's permeable (I don't care what any manufacturer says, they are still permeable). Al. waste tanks will be gone in short order as will S.S., but it will just take longer.
The solution for waste and water tanks? An Al.(or steel) tank with the interior coated with epoxy ......yep, that's what all my big boat tanks were made from and the diesel tanks were just straight Al. or steel depending on the hull material. Also, something like a Lectra-San or a 5 gallon bucket is another approach to the waste tank dilemna and stink. I kid you not, when I inspect commercial fishing boats for the USCG, a 5 gallon bucket is acceptable! But they are going off-shore and state regs may pre-empt the Feds. But it's considered a 'portable waste storage device' and I have to pass them on it. No kidding. Just don't try it and mention my name.....LOL.
Moeller advertises that their tanks meet USCG and ABYC standards, etc, so not sure I'd fail a safety check, though now I will check with my local auxiliary to be sure.
Ok, just to correct the perception that the CG discourages plastic tanks, here is a link to the USCG recreational boatbuilders' handbook, which lists federal laws for boat building standards: http://uscgboating.org/safety/boatbuilder/index.htm. Polyethylene is not a prohibited material, at least for recreational boats - the tanks just have to meet all the standards, including pressure testing, etc.
There is plastic, and there is plastic...
Anyway, I have read the previous threads and get the gist - aluminum seems the popular choice - just wanted to see if anyone had any actual experience with the polyethylene tanks. I've heard no negative experiences so far, but they don't have as much history.