Is a 20-gallon tank all right for a green terror?
I am assuming here you mean a Green Terror Cichlid, in which case the answer is no. It would be okay for awhile if the terror is a baby, but in my experience he/she will outgrow it quite quickly. I would say the minimum tank size is 50 gallons, as these cichlids get quite large. They are also quite aggressive. My terror killed his mate and 2 Oscars within 2 weeks. Hope this helps!