I use a 20 or 29g tank. Heater, top, darkened on 3 sides with PVC pieces inside for cover. No substrate, no filtration. Maxijet powerhead aimed at the surface for movement and aeration.
currently i have a 20 gallon on a 75reef
im enlarging that to a 50 gallon- combination of rubbermade containers inorder to have a large dsb area. the bigger the better imo.
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Beerbaron, I am not sure if incorporating DSB into a "quarantine tank" is a good idea. Everytime you use chemicals to treat new additions, you will likely kill everything in the DSB as well.
Back to the original topic. I don't have one that is always setup. When I need one, I use empty IO salt buckets instead (Got plenty of those around). Fill it with water, add a heater and powerhead. Keep the lid closed most of time. Monitor ammonia/nitrite level, and change water when necessary.
I have a 40g breeder setup as a quarintine tank. It has PVC pipe and some elbows of various sizes, a heater, a powerhead, a Prizm (extra from old tank), a thermometer, and eggcrate on top. The reason mine is so big is because thats what I had in the basement and I have a few big fish(angels) in the the future plans.
HTH