Hello and welcome. My name is ***** ***** I'm a biologist with a special interest in fish. For now, your fish are fine. Your water parameters are excellent. Both the comets and the wakin are goldfish. They can all do well in either a tank or a pond. Your wakin is not a koi, and actually all goldfish are related to koi, but not the same species. The wakin looks quite healthy in the photo you sent. A fish may hide if the other fish in a tank chase it or harass it, so you may want to keep an eye out for such behavior.
So, it appears you were given some misinformation. What you do need to do though, is start saving for a bigger tank. Your fish will outgrow their present one, and you will not be able to maintain water quality when that happens.
It used to be recommended to have one gallon (about 4 liters) of water per inch of fish. That still holds true for tropical fish, but not for goldfish. There are various recommendations, ranging from 2 gallons (8 liters) per inch of fish to 30 gallons (about 113 liters) per fish, regardless of size. I’ll share my own experience with 12 inches total of goldfish (3 fish). They are in a 90 gallon (339 liters) tank. I have an undergravel filter, a hanging filter, and a powerful canister filter. It takes all three of those filters to maintain perfect water quality. Regardless of tank size, frequent water testing is always needed.
As long as ammonia and nitrites stay at zero, and nitrates below 40 ppm, your fish will be OK. When the numbers go up, it will be time to buy a bigger tank. If you have more questions, let me know in a REPLY. I hope your fish will thrive for a long time to come.
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