Help choosing a new filter.

[Meyer Jordan]

Thanks for the info.
I frequently see 1 adult koi per 300 gallons cited.

Without looking very hard, you can find those that espouse 1000 gallons/adult Koi.
What really needs to be considered is what is needed to maintain healthy fish? Basically only two (2) things: acceptable water quality and exercise space.
Water quality is maintained by proper flow rate and adequate available bio-conversion. A point often missed is that a fish is affected only by the quality of the water IMMEDIATELY surrounding it. Large ratios of water volume to fish is completely unnecessary .It matters not the proximity of other fish or their numbers. In fact, Carp/Koi are mostly shoaling fish, so they naturally crowd. Common sense dictates that Ammonia levels will be higher in a shoaling scenario. Maintaining a 1.5 to 2 times per hour water turnover rate, minimizes any chance of Ammonia levels reaching a toxic level.
Exercise space should adequate for fish to maintain proper muscle tone. Horizontal space should be given prime consideration. First, because the majority of swimming done by a carp/Koi IS horizontal (in their natural habitat (carp will spend much of their time in the shallows in both Spring and Fall) and, secondly, as an added benefit, horizontal space equates to water surface square footage which is all important for oxygenation. Depth is secondary and is dictated by the prevailing climate.

 

[dieselplower]

2 x per hour is no guarantee of anything if the filters aren't up to the task. You want to put a 3 foot ling fish in 100 gallons? That's just cruel.

 

[Meyer Jordan]

2 x per hour is no guarantee of anything if the filters aren't up to the task. You want to put a 3 foot ling fish in 100 gallons? That's just cruel.

You must have adequate available bioconversion in addition to the turnover rate.