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Gallons per hour question

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Most forums articles i have read indicate 10 times the gallons per hour turnover for goldfish. (e.g. 600 gallons per hour for 60 gallons and so on).
Recently i had someone tell me that the faster a filter turns over the less contact time to media and the less efficient it becomes.

By that logic we should really reduce the gph and just have a large surface area for the sponge. 

I have a little more than 10 times the gph on my tanks and i see value in that, but would like to see if anyone can weigh in on this.

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The nitrifiers do their work virtually instantaneously.  You send the ammonia/nitrite to them, they oxidize it.    Contact time is an old idea which has been pretty well discarded.    When you use HOB filters with tiny (relative to the tank volume) space for biomedium, you have to use high turnover to avoid leaving ammonia in the tank.   The media volume and the turnover rate of aquarium filters are designed to work together for maximum efficiency.

A turnover time of 10xVol per hour also picks up most of the solid waste produced by big fish like goldfish.  If you use 5x vol/hr, as the manufacturers recommend, in a goldfish tank, you should plan to vacuum the bottom every day, because there will be fish poop there,  and also prepare for cloudy water.   If you had a heavily planted tank with a community of small tropicals,  as little as 4xVol/hr would work fine.

If you use a canister, with larger media space, you can go with 5-7x vol/hr. since the larger filter treats a larger fraction of the system water at any given time. 

If you build a pond filter with a volume of at least 1/10 the tank volume, you can go with 2X volume/hour.  

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