If it as simple as installing a new bulkhead then you shouldn't need a temporary tank, livestock and water could go in a big rubbermaid or 2 while you install a new bulkhead and thead in new fittings etc.

If you are comfortable having the livestock ( I don't know how much you have) in rubbermaids with pumps running for a few days I would highly recommend getting an overflow installed. Wont have to worry about draining again.

Orca can be used under water :)

It looks like you have a nut on the topside (underwater). A nut on the bottomside and also your barbed fitting is threaded into the pipe coming down? Is that correct?
You could try using a pipe wrench on the top (not a dirty old shop one) :neutral:, as long as the bulkhead has a rubber gasket and not silicone (as that would probably make it worse). The bottom nut looks like it could only be tightened using some McGuyver skills (a custom tool that would look like an angle grinder wrench with long posts). If its leaking from where the 'barbed' fitting that screws in then you could use a balloon or condom and a tight rubber band to stop the flow while you spun that out and Teflon taped or siliconed that connection?

This may be your best option if used from the topside

It doesn't look like the right type of fitting to use to begin with?!

The gasket is on the wet side it's a smooth flange it's thread on both ends

Verify your bulkhead isn't cracked

Gasket could be shot or God forbid even a small crack

That whole setup is just scary IMO

Could be silicone too which is even scarier

As much as it's a PITA to have to tear your tank down, I suggest you do it and put in a new bulk head fitting and rubber gasket, BTW that bulk head in the picture is not a nut on the top water side. I've seen those before they have a full length female thread, just another style.