Looking for a 100% non toxic acrylic sealer?

im building a castle for an aquarium, the castle is made out of styrofoam, and im grouting it with non sanded polyblend grout. i need to seal it and since its going underwater, i need a 100% non toxic acrylic sealer. i was looking at a video on lizard-landscapes.com and he uses a product called Shields-all.. but to ship it is too expensive.. im wondering if there is another sealer that works just as well. im also thinking of mixing aquarium sealer and water .. let me know if you have any specific sealants that will do what i need i've checked Rona, and home depot.. theres only 1 lowe's in my area so i havent checked there yet