Just about any healthy aquarium will contain inactive diseases and parasites. If the fish are healthy, their immune systems will protect them. They get unhealthy and unable to protect themselves when the aquarium gets dirty or produces harmful chemicals; almost every time, those things are caused by over feeding, over crowding and/or lack of proper maintenance. Regular, partial water changes are necessary because an aquarium is a closed ecological system. If the biological load is not very big, the plants and bacteria in the aquarium can cope with most of the waste materials. Partial water changes are the only way to get rid of all the waste products and dangerous chemicals.
Since you are going to start over, you could add a cup of gravel from the established tank plus the new gravel and some live plants like hornwort and java fern. Before adding fish, check the water once a week for a few weeks to make sure there is no ammonia or nitrite. When conditions are right, add some snails if you want and one pair of inexpensive live bearing fish like guppies or red wag platties. When you add them, use the drip acclimation method. If you've done things right, you should end up with a tank full of fish in six or eight months. Be patient, and let nature do all the work.
I'm glad some of our suggestions have been helpful. Please keep us posted on your aquarium adventures.
