Small tanks are a challenge to maintain. The water quality can foul in a matter of hours and stress the fish. This will weaken the immune system, and parasites that live dormant in our tanks can infect the fish.
Good idea to go with a much larger tank. They're much easier to maintain and if there's a mistake in tank management, there's enough water to dilute the problem. Large, weekly water changes will keep the water chemistry safe for the fish and the plants.
As for the old decorations from the diseased tank, you can soak them in a bucket of treated tap water and add a couple of tablespoons of standard aquarium salt. Most, if not all fish pathogens are unable to tolerate even a trace of salt in the water. Soak the pieces for a few days and then rinse well.
Remember to keep the water in this new tank clean. Large, weekly water changes of half the tank volume will guarantee safe conditions for your fish.
"Fear not, my young apprentice. Just change the tank water."