A 20g tank will work for most normal size fish. Bare bottom is good. Use big pieces of PVC
as hiding places. Heater is good if you need it in your area. Filter is good or at least a power head to circulate the water. Having something like a sponge that you can add from you DT is good for beneficial bacteria. Watch the parameters closely, do water changes as needed.
Some people treat any new fish as if it has something already, but I've always observed and treated as necessary (which luckily has been never). I figure hypo and copper can cause stress and if they do not have anything, then there is no point.