I kept Comets for years while trusting the advice given in my LFS
brochure No mention was made about the required tank size. I thought a 20g tank was fine for them. Sadly they would usually last about 3-7 years before dying. I know now that if they're in an adequate tank they can live for 20 years. If they are in a less than adequate tank they won't last as long or live very comfortably. Always research the requirements of your fish if you want.
Most common mistake is letting buckets overflow during water changes.