Yes, you need to do it... No, it's not okay to overload your pond because all your fish have names.
Remember, each pond can hold one 10" koi for every 150 gallons of water "with a good filtration system". When a koi reaches 18-20" the rule of thumb is 500 gallons of water for each fish.
- Test your ammonia daily if it exceeds 0.25 on the vial until you have it under control. After you have tested it remove 20% of the water and refill (if you have city water you should be adding a metal & chlorine remover). Test again to see what it is. If it hasn't gone down remove another 20% and retest until it's under 0.25.
- Over filtrate.