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[Beverly]

Al,

Do you mean "Koi"? If so, these fish grown to 24" as adults and are really meant to be kept in outdoor ponds. A 120g is going to be too small for them over the long term.

However, I would use at least two AquaClear 500s on a 120g. Fill the media chambers with foams instead of the carbon and other inserts that come with the 500s. Thoroughly clean the foams weekly in outgoing changewater.

Kept an indoor pond many years ago for four flat-bodied goldfish. Started out with 1 1/2" substrate, but found it was tough to get all the crud out. Finally went BB and crud siphoning was easy and way more efficient.