The cheapest way that will work and last will be just to fiberglass it.. Look in the yellow pages for a fiber glass shop and see what they charge a gallon for resine. my tank was 8x3x3 and i used 5 gallons, but that was multiple coats..
You can use sweetwater paint (http://www.aquatic-eco.com/categories/Paint--Expanding-Foam/169/0/ ) which dries like a sheet of ruber, but i wouldnt use it by it self..
why not just run some real thin plastic sheet? When I say sheet I mean the ridged kind not the kind on a roll. then just seal then the seams with how pressure silicone. would be very cheap and with plywood the plastic doesnt need to have any structural needs
This is what I am using to build my tank. We used a "bite coat" first to seal the wood and then came back later with a second coat. Normally it is a plain resin color but we added black to it.