A hospital tank is a separate tank used to treat a sick fish away from the community, so medication does not harm other livestock, plants or filter bacteria.
Many medications are hard on shrimp, snails, scaleless fish and beneficial bacteria, so treating in a dedicated tank protects the main system. A quarantine tank often doubles as a hospital tank.
Keeping a spare cycled sponge filter running means you can set one up at short notice when a fish needs help.