My tank with fish in it is leaking!!!!!!

[MultipleTankSyndrome]

To reply to everyone, thank you once more for the tip on the silicone taking more than 2 days to be ready because you were absolutely correct. It has been curing for a full 6 days before the water and fish went in, the vinegar smell had not gone away at the 2 day mark and I thought to just leave it when I remembered the advice I got here.

One more thing I was too overwrought to mention earlier but probably should: The black kuhli loaches presented both the easiest and hardest fish to catch in the tank.
A surprising amount of silt was still in the sand, which allowed the kuhlis to camoflauge with the sand that I was unable to remove! And that's not even counting their small size (meaning they could slip away from the net's pathway quite easily) and surprising speed.
One black kuhli loach was so well camoflauged, in fact, that I could only tell it was still in the tank by signs of it darting away from the net (like sand and silt being stirred up where it was). So I focused my attention on the remainder of the fish to the point where it was the last in the tank, and what happened next was almost the equivalent of an apology or consolation for all my effort.
The second-last fish I caught before the kuhli loach was the pictus catfish with the damaged fin. Un-sticking it from the net was not in the slightest fun, so I was really hoping for an easier time with the kuhli loach. And I had it, because when I headed back to the first 473 liter from the second, the kuhli loach was sittig at the front of the tank in plain slight.
It also showed no resistance when I scooped it up and transferred it to the other tank. So that was a great way to end it.

 

[Fishman Dave]

For future reference, best not trying to catch and move Pictus cats with a net. Your local 5buck store, dollar store, hardware store or similar will have either a plastic sieve you use for draining cooked food , or similar plastic storage box you use for condiments or spices in the kitchen (those here in the uk even come with a handle) and those are absolutely ideal for cornering Pictus and transferring them quickly to bucket. Yes they flap about wildly but having cut pectoral fins out of so many nets it is cheaper, less time consuming and less stressful for the fish with one of these.
And like I say you can even dual purpose it when it’s not catching Pictus.

 

[MultipleTankSyndrome]

After a much longer time period than I'd like, the original leaky 473 liter has been resealed. Finally.
Now the only remaining step before this leak fiasco is over for good is to fill it and move the fish back once the silicone sets (planned for Feb. 13). They'll certainly be happy about it, as they can have their sand substrate that couldn't come with them to the 2nd 473 liter back!

As it turns out, however, there was more to the tank's problems than initially met the eye. Not only was the tank badly positioned on the stand, the stand's beams were bowing and the silicone was full of bubbles, indicative of old age. So this was a tank that not only had worn-out silicone, but was badly placed on a bowing stand too!
To fix the bowing stand, some supports were put between the tank and the area where the stand's beam bows. According to the repairman, they will stimulate the supports of a straight beam in the places where the stand bows, and it's easy to see how they would do so.

 

[MultipleTankSyndrome]

Final update: The fish have been moved on February 20, a full week later than planned, because the vinegar smell did not go away on the 13th. They now have their sand back, and although they still appear to be getting used to their old tank, they already appear much more at ease than they were in the temporarily bare bottomed setup.

Thank goodness it's all over now.

edit: Forgot to mention, I now own a 15 meter Python unlike when I had to move the fish out of this tank. That allowed me to drain the tank until almost empty and scoop up all the fish in about 30 minutes as opposed to last time's 5 hours.
I didn't have to deal with unsticking the pictus catfish from any nets either, courtesy of Fishman Dave's excellent suggestion. Those utensils absolutely excel in catching and moving pictus catfish.