Because I bred plecos for sale, I needed to collect the offspring from the tanks. Plecos are hard wired to hide which makes them difficult to catch. To do so I had to completely break down the tank to be able to catch the fish. When I say break down I mean I removed everything from the tank except for the substrate and heaters. Further, catching the fish is a lot easier the less water there is in the tank. By removing 1/2 or more of the water, it reduces the places the fish can go.
Of course once the needed fish were removed I had a unique opportunity. I could execute any sort of rescape I wanted.For the most part I wanted to return things to a similar state. But I also had to make some changes. As the breeders got larger, I needed to upgrade the caves to larger sizes. In some cases I wanted to add substrate to a bare bottom tank or even to change from small sized gravel to sand.
I also needed to do all of the above on the same day so that the tank could be returned to functioning and the fish ould be back in it. All of this was complicated by how well plecos can hide. I lerned early on by killing a few fish that I needed to move all of the decore to a holing container with water at the proper temp and good aeration. I had to do this catching multiple times across multiple tanks every year.
So the method I evolved was to use a 16 gal. (give or take) rubbermade tub with a heater and air stone to which I put everything I removed from the tank. About 90% of the time I would find a few fry and even an adult fish in the Rubbermaid tubwhen I was putting everything back into the tank. My method had several advantages.
1. It kept all the contents of the tank moved to the tub in water. There is a lot of bacteria on the rocks and wood as well as the substrate in tanks and keeping these alive requires they also be kept wet..
2. Sometime real life intrudes on out fish keeping and if I had to stop working in the tank I knew that the fish and decor was safe in the rubbermaid, even if this was overnight. And sometimes I wanted to pause things so I could eat.
3. It is a lot easier to catch fish in the tub than in another going tank filled with decor etc.
4. The fish that might be in the rub had plecnty of places to hide and they had no other fish which might result in conflicts.
There have been times when I wanted to change substrate in a tank. What I earned was the easiest wat to remove most subtrates is with a siphon. I could suck out the substrate to a bucket and then return the water to the tank by pouring it through a fine mesh net to prevent junk from being returned to the tank. Then I could suck out more substrate without drastically lowering the water level.
Another trick I learned was i could add substrate to specific areas in an established tank using this trick, I would take the long tube part of a tank vacuum and I could pour sand of gravel into the top of it which was above the surface and have it come out the bottom of the tube exactly where in a tank I needed it to go. This was quite handy not only for having the substrate going where I wanted it it, but this also prevented it from landing on plants ot decor where I did not want it.
The above techniques meant only one tank was getting disturbed. If I had parked the fish in another going tank it had the potential to mess up that other tank trying to get the parked fish back out. It was enough that I had to disrupt the going tank and I did not want to have to cause a havoc in another tank as well.
Very recently I had a 150 gal. tank develop a leak, I needed to move the entire contents of that tank to a smaller 125 gal tank. I did this over a weekend and at the end of day one I had most of the decor and live plants moved over but very few of the fish. I left the canister on the 150 with some wood and a big cave in it over night. I moved the AquaClear 110 and the H.O.T. Magnum canister onto the 125.
So, instead of using another of your tanks to park the fish and since you cannot get another tank, I would suggest your best bet is to get a decent sized rubbermaid and use that to park things in while you do the work in the tank you need to change around. The tub will come down as soon as you finish which should be one day or maybe as many as two.
I bought my Rubbermaid Sterlite Storage bins at Target. Depending on the size you choose you should have to spend between $10 and $15 which is a lot less than another tnak would cost if you could buy one. Plus, your folks may want yo use if for otjer purposes when you are done if you figure you wont need it in the future. Be sure you get the lid as well as the bin as lids should come with them but are often stacked seperately from the bins which they like to nest to save space on the stores shelves.
You do not need the kind which have a latchable lid, the snap on lid is just fine. In the store buying singles is easy, online they tend to shows them in batches rather than singles. The pic below is a 56 quart example which is a 14 gal. bin. Mine have white lids but they sell them with different color lids.
