you can use sleepers just over lap them on the ends and then drill a 32mm hole in each corner then put a 32mm peg in each hole that will hold them together you could put 2 in each corner to be safe and if you put them about 10mm under the surface you can get some wood resin and mix it with sawdust and it will dry like a knot in the wood and the tada you have a frame hope this helps.
you dont need a UV light i have a 450 gall ponds with no uv lights and its as clear as tap water just good curculation and plent of oxygen and a few snails will do
I have a outdoor pond, I made it out of treated 4X4's, liner, and underliner.
It is 12'X8' and 3' deep, I have a UV filter system in it.
It is shaded in the summer, and I put a live stock tank heater in it for the winter. That turns on and off by temp. It keept a hole in the ice all winter.
Mike