Just posted in your "ich" thread, so obviously that issue needs resolution before any further stress like moving tanks.
As you cannot set the new tank in a different spot, you will need to move the existing (obviously) but that means moving the fish to a smaller temporary tank. I always do this so I can take my time, and if the fish need to be in the temp tank for one or more nights, it is not a problem. Use most of the water from the existing directly into the temp, add some decor and plants from the existing, especially floating plants which are very important here for shade, loess stress and water quality--they are ammonia sinks. Then drain, take out everything left, and move the tank and set up the new one. You can use the substrate from the former, with more added probably as it is bigger. You have cories so you want sand, and quality play sand is ideal. In the UK, Argos Play Sand is good, others here use it. Arrange the wood/rock, fill the tank with water trying not to disturb the sand too much (running the water into a large bowl so it gently overlaps works). Make sure the heater and filter are working, and I would leave it overnight to be certain. In the morning, when ready, net the fish over. Temperature must be the same. GH and pH presumably will be the same.
