The Koi House
The building is 16' x 13' with the walls only 6' high. This kept the overall height down for heating purposes. The entire framing is of PT lumber…so no worry about rot, etc. There are 14 windows in it, along with 4 skylights. This has given me a lot of passive solar heat. I also don't have to worry about lighting for the fish.
The main tank has an inside dimension of 11' x 4' x 5' deep. Three feet of its depth is in the ground…in what we locally call ''hardpan''. As much as it's dirt it's rock hard! What a pain to dig in!  The other 2' are made of  2' x 6' framing. Then the entire inside is sheathed in ˝'' plywood.
The liner was then installed over this.  There's one 4'' bottom drain in the tank along with 2 TPR's for current. The bottom drain is made out of a toilet flange and the TPR's are eyeball jets for a swimming pool.
The filter for this tank is made out of 3-55 gallon barrels,
with toilet flanges installed as bulkheads between the
barrels  …the 1st acts as a settling chamber,
the 2nd as a mechanical filter and the 3rd as a sump
for the main pump and for the pump that feeds the TT.
The bottom drain has been set in concrete also to prevent any movement
The piping on the filter is the same size as on the main pond…that way I can use the AquaFlo pump on both systems.
The building is being heated by de-humidifier and the tank heater, along with whatever solar heat I'm getting.
At any time it's about 80* in there and the water temp is 75*.  The fish will be nice and happy for the winter and I can relax and not worry about them.