PVC pipe and other plastic pipes do not conduct heat well, this is the reason I stated copper. Others also use titanium in much smaller lengths but thats way to expensive. To address your heat issues and other issues I'll continue.
First, a 300 gallon pond will not be anywhere near large enough for several type of sturgeon. Second, enclosing your fridge in a box will not help it cool though it may protect it from the elements. You'll need to do a few things if you want to keep a sturgeon in a pond. One, keep it well filtered and oxygenated as sturgeon prefer well oxygenated water. Two, make a shade cover for the pond to prevent the sun from heating the pond during the daytime. If you made a large enough pond that was deep enough and covered with some sort of shade you may be able to keep the daytime pond temps in the low 70's or upper 60's.
As for chilling, if you are going to try to build the fridge chiller do it right and use copper and a lot of it. You are dealing with surface area and time of contact with that surface area. A slower pump will help will cooler water temps. Good luck with your endeavor should you try it.
First, a 300 gallon pond will not be anywhere near large enough for several type of sturgeon. Second, enclosing your fridge in a box will not help it cool though it may protect it from the elements. You'll need to do a few things if you want to keep a sturgeon in a pond. One, keep it well filtered and oxygenated as sturgeon prefer well oxygenated water. Two, make a shade cover for the pond to prevent the sun from heating the pond during the daytime. If you made a large enough pond that was deep enough and covered with some sort of shade you may be able to keep the daytime pond temps in the low 70's or upper 60's.
As for chilling, if you are going to try to build the fridge chiller do it right and use copper and a lot of it. You are dealing with surface area and time of contact with that surface area. A slower pump will help will cooler water temps. Good luck with your endeavor should you try it.