at two inches, 1.5 inches a month, 4 months 6 inches, plus the two inches its at now, so 8 inches i would say 4 months MAYBE 5 months at most...
Koi generally don't do well in small tanks, regardless of size. That 20g will last you 3 months, +- a month. Koi usually get to 2'.. I've seen smaller, and I've seen a 4 footer but 2 feet is normal.
Sent from my Toaster
If their are no other tankmates, just 1 koi, weekly water changes of at least 50% are performed, and he is fed sparingly 1x a week max and its a 20g long then probably 6 months.
Feed it once a week? Kind if a cruel way to put off the upgrade font you think?
Water temp and quality will play as big a role. Sorry, no way your going to properly house a Koi in a 20 gallon for a year.
Sent from my iPhone using MonsterAquariaNetwork app