It’s always a good idea to start small and add fish gradually. Here is a rule of thumb to use as a guideline to avoid overstocking and ensure the overall health of your fish:
• For small fish like goldfish and comets, 1 in. of fish for every 3 gallons of water.
• For Koi fish, 1 in. of fish for every 5 gallons of water. Koi fish have a faster growth rate, so you may have to remove a few of them at a certain point.