High organic loads consume oxygen and are a haven for bad bacteria such as aeramonas and pseudomonas. Goldfish tend to be hardier than koi and can survive in poorer water quality. To build a koi pond with proper filtration is an art in itself. That's why it's very important that whoever builds your pond, if you are wanting a true koi pond, consults with or is a koi pond professional. Proper filtration is certainly necessary.
Good bacteria is needed to complete the cycle of changing ammonia into a useful and harmless state.
Good bacteria is housed in filter material. A Pump Pre-Filter catches the solids before they go into the pump where a load of debris can do a lot of damage. A pre-filter does nothing more than filter the large debris from the water before it travels into your pump. It does house some good bacteria but not enough, in most cases, to do the job. A Bio-Filter houses the majority of your good bacteria.