Backyard Aquaponics
A place to discuss aquaponics

I have several free sources for large polyethylene tubs and I was wondering if polyethylene was a safe plastic. The tubs I am talking about are encapsulated foam(used to support a floating dock on the lake) and many of the dock repair companies just hold on to the broken ones. the top can be cut off and the foam removed. My only expense would be to pay to recycle the foam inside. Here is a link to a company that sells them.
http://www.dockbuilders.com/dock-floats.htm
We also have another source of tanks that I am not sure if I will be able to get for free or not but they are much larger. these are used to lower and raise boats out of the water, I think it is also polyethylene.
http://www.polylift.com/product_styles

Is polyethylene safe to use for growbeds?

I plan on framing some wood supports to prevent the beds from bowing out, I also plan on putting some sort of barrier around the sides to prevent the black plastic from cooking my beds in the sun.

yes polyethylene should be ok.

how hard is it to get the foam out?

Pretty easy, cut the top off with a reciprocating saw and pull it out, the tubs are tapered a little. I plan to make a hot knife cutter to slice the foam into slices and use it to insulate a shed.
I volunteer in a shoreline cleanup program so I have pulled plenty out of the lake that have broken off from docks. I even have a couple on hand. I have talked to a dock builder that has a bunch of them ranging from 12-20 inches deep.

Food grade PE is certainly safe to use, as for non food grade, there must be a reason it doesn't make the grade, but I don't know which partcular chemicals might leach out that are of concern for you or your fish.