Does Anybody Really Know The Answer

Yes, not a simple question. BTW Is there any way you can elevate the wet dry system a bit? If you put a healthy number of plants in your tank you reduce the amount of work your filters have to do. If you understock your tank you are also miles ahead of the game. If your tank is on the cooler side the water has the ability to hold more oxygen so the bacteria in your tank can do more of the work - filters have to do less. If you have a conservative feeding plan your tank will be less dirty and will not need as much filtration.

Just because your pump is "moving", say 500 gallons per hour and your tank has 100 gallons does not mean that each gallon in your tank will be moved 5 times. If you have hob filters you may be circulating 50 gallons of your tank 10 times. If you had another power head or pump inside the tank to circulate the water it will help create stronger current in the water so the water will circulate better and give you better access to all 100 gallons. Also some filtering systems allow some water to pass by without being filtered, so even if 500 gallons of water is passing through your filter box maybe only 90% of it is really being filtered , the rest of it is just going around one or more portions of the media. (Note: I am pulling numbers out of thin air just to illustrate a point.) That is one reason why I always:

1. if the tank is larger than 20 gallons I always put a ph or pump in the tank just to circulate the water. Usually I use pvc to create current from one end to the other.

2. use 2 kinds of filtering systems. My first choice is always a rugf (I don't have a lot of experience with plants yet so I am not sure how well the rugf will work with heavily planted tanks - some people say no problem) and I draw the water from the bottom of the tank as well as from the middle (top to bottom) of the tank.

3. even though I prefer to keep the hardware out of the tank I still prefer to put the heater in the tank, mounted horizontally in the bottom middle back of the tank to let me use convection to circulate water in the tank.

4. I never had the money in the past but if I had the funds my second filter would be some kind of canister filter because there is less chance for water to bypass the media inside the canister. It can still happen, especially as the filter media gets clogged or old but, at least 10 years ago when I was reading on this topic, the canister filters had a better % than HOB filters...but they are more expensive than the hob filters. I have never tried it but if I can get enough $$ together to do another big tank I am going to try a fluidized sand bed.
 
I think its been pretty much covered. I'd agree that 4-5 times is BARE minimum.

I also go for 10x per hour.
 
A little over 11 times on the 80
A little over 13 times on the 45
A little over 12 times on the 29 {With the AC-50, QT tank filter} ......:)
 
I like 10x as a goal...

My 75G (probly 65G actual water) has 3 Penguin 660R's @ 170 each for 540 GPH into my RUGF, + a Whisper 60 HOB rated 300GPH (probly closer to 100-200)

So I'm ~10x...BUT...I have 8 big GF and a 7" plec in the tank. yes, I need to rehome a few GF :)

My 20H endler tank has a 660R driven RUGF + a whisper 40 rated 200 GPH
That puts me ~ 13x on that one with the whisper 40 at 'half' flow.

30g..soon to be 40g platy/cory tank...one 660R and a whisper 60... 10x
...may add another 660R when i go to the 40G.

On any of these...if one pump fails/jams, I still have acceptable filtration til t'other can be fixed.
 
This is one of those 'measurements' in aquaria that are pointless.

You could have the 'optimal' turn over rate, but if you don't have enough media, or have too many fish, or aren't doing enough water changes, then the purpose is defeated.

Use big filters on big tanks, do at least 50% WC's once a week at least, and stock your aquariums to the capacity of your filtration/space, and that's all that really matters.
 
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