It's not necessary to keep a quarantine tank going when it's not being utilized for a new or sick fish.
Probably the easiest thing to do is have a HOB filter that uses a simple sponge filter media w/o activated carbon, and keep the sponge in the sump of your DT. When it's time to quarantine a new arrival, or remove a sick fish to the quarantine/hospital tank, just take the seeded sponge from your sump and put it in the HOB filter on the quarantine tank. After several weeks of use, just toss it, and put a new one in your sump to be ready for the next go-round. The quarantine/hospital tank, along with all equipment and PVC fittings you provide for hiding areas, should be washed out with a gentle bleach/water mixture, and it'll be ready to go.