Initial construction - fish tank

It gets too cold here in the wintertime to maintain the fish I wanted to keep so I really need to build a super insulated glasshouse for this project . However I couldn't decide on how to do this despite many versions of plans in my head.
So I decided to modify one end of my big shed to accomodate a trial setup- ended up a bit big for a trial but I think it will work well enough to determine if the construction of a dedicated greenhouse will be worth the effort.
Without really giving the whole thing too much thought I decided to simply copy what has been show to work by Joel Malcolm in Perth. His system is outside but apart from that minor detail my system is fairly similar.
As stated elsewhere we are entirely solar powered (actually partly wind powered but that is really a small part of the total) and so the system had to be as energy efficient as possible.
So the tank and grow beds are, or will be, heavily insulated to conserve as much heat in the winter as possible.
I also had to minimise pump usage so the system is a flood and drain one where the pump runs for between 6 and 9 minutes in the hour and drains back via gravity. IE no sump pump.
I have recently changed the pump but I will leave the details of that to a separate post.
So the fish tank had to be below the level of the growbeds if a gravity system was going to work. It also needs to be well insulated so one really big hole was needed- 2500mm across and 1400mm deep.
Took me 4 days to dig the hole . Basically dug by hand- needed the jackhammer to break up the dirt and used the tractor bucket to move it outside. Not sorry to have finished that little job!
Then put 20-30mm of crusher dust in the bottom , leveled it off and added 25mm of expanded polystyrene over the bottom. Lowered the tank in, packed 30mm of expanded polystyrene around the edge and backfilled with more crusher dust.
The tank was then filled with water and left to sit while I tried to work out some way of heating the water. This part of the plan is still in thought/planning stage as the initial plans were thwarted by the fact that copper pipe in contact with the fish water releases copper which can be very bad for the fish. Continuing cold weather basically put the whole scheme on hold until it started to warm up in September.