sorry to repeat but the tank is very small for those fish i have a 3ft tank with only four goldfish in it and three filters. you really do need a much bigger tank i would suggest at least a 3ft and you will also need massive amounts of filtration, as goldfish are very messy fish.
koi can grow very large and even your comet goldfish can and will get to 12"
when you feed flake food to goldfish it can cause problems with the swim bladder so soak the flakes in some tank water before putting it into the tank.