I have inherited a small pond two years ago, appx 700-800 when full, but has been leaking for two years, owners were cheap and used a plastic mesh type liner and a horrible job building the structure. Now the leak has gone dramatic (manageable before), it must be in the bottom or near so my water level goes down to about 10".
Stats: Texas, so a lot of flip flop temperatures now, water temp the other day was 41, but will rise to 50+ easily with warmer days in forecast.

4 medium sized koi, and approximately 15 goldfish.

I use Pond Prime constantly as a de-chlor when adding water, and a fliter with de-chlor filter with carbon etc...also add bacteria.

Bio filter by Laguna and pump, just sits in pond and goes to a waterfall.

I just purchased a 4000 gal aerator and placed in pond as I knew I would need for temporary housing to fix pond.

Sorry for the length but I would like to do this painless for the fish and myself, I need to setup a temporary house, which I am going to get a 170-300 gal metal cattle trough and use. My plan is to pump the little existing pond water into it, add the aerator, and put in the Laguna pump with hoses going to the biofilter and let it dump back in.

Then rebuild my pond with a concrete/mortar base, do a spray in coating.

I mainly want to know what to do with the temporary and if I can get by with the smaller tank of 170 gallons?
Is my plan to pump the existing water into it ok? Do I need to heat or just let it go? I have a complete water test kit. Also when I start pumping from main pond into temp tank, I will have to move fish immediately into temp tank as the main will not have any water in it. Or should I just use Pond Prime and add tap water into temp?

I will investigate how to setup the main pond after rebuild. I am budget limited and respect the what I should have vs what I can afford. Right now I cannot afford any of this (****** that previous owner was a cheapskate), but will buy a temp tank, and rebuild the pond to the best of my ability. I will make it larger at appx 1000-1200 gallons (there is no more room than this).