I built the ply wood box with 3/4 plywood and tons of screws and wood glued the joints. Reinforced the top with. 3 plywood strips......will post some pics soon. It seems solid but I'm going to reinforce the sides anyways when I put it in place and frame the wall around it. So for the lining I think I'm gunna use this stuff made by pol-wall made by pas-tex. Its food safe so non toxic for the fish and seals with silicone. I found it at menards for like 13 bucks for a 4x8 sheet. More to come
I am interested in knowing what you found out about this product and silicone. I went to the company web site and found that they sell a special purpose sealant formulated for adhesion to recycled plastics. I could not find any more specific information. This could be a hybrid type silicone, but that is just a guess. Definitely test! Also be aware that some sealant / substrate combinations behave differently when submerged - so a dry test does not always tell the whole story. You may want to consider investigating hybrid or polyether sealants for this application.
