If you try to stick to acrylic to pond liner, that is usually plastic, the seam will rip out long before the tank gets full. For this, you are going to need to look at solvent-weld type glues... I'd probably start with the glues designed for joining two places of liner in ponds, as it's plastic to plastic bonding you are doing. There is also solvent pipe weld, but try it on an off-cut first as it can distort some plastics after application, while it sets, IME but I cannot remember if the Acrylic or Pond liner type plastic are the types is distorts