Pump Output vs. Max Flow Rate of filters

[Dork Fish]

There seems to be conflicting information online about the difference between 'Pump Output' and 'Max Flow Rate'. For example, the Fluval FX series of filters have both stats listed on their website. The FX4 says it is rated for up to a 250 gallon tank. The consensus in the hobby seems to be that the water turnover rate should be 5x the size of your tank (in volume). Or ideally 10x for heavy bioload/planted tank.

For a 75 gallon tank that number would be 375 GPH (with media) or 750 GPH for planted tank. For a 250 gallon tank it would be 1,250 GPH or 2,500 GPH for planted.

As an example, Fluval rates the Pump Output of the FX4 at 700 GPH and the Max Flow Rate at 450 GPH. This appears to be only half the ideal Max Flow Rate of 750 GPH for a planted tank, meaning you would need 2x FX4s to meet the ideal.

The Fluval X07 series doesn't even list the Max Flow Rate, only the Pump Output.

Is the Pump Output or Max Flow Rate more important? Am I missing something here or would you need at least 2 FX4 for a heavy bioload/planted tank?

For Fluval website:

[Biotope Biologist]

Max flow rate is most important stat. Pump output is how much the pump can push directly around it. Flow rate takes into account efficiency losses through tubing, height gain etc. 

 

Turnover rate rule is a very loose rule. In smaller tanks it’s much easier to achieve 5x turnover because well that’s only like 50-100 gallons per hour. But it’s multiplicative. If you want to turn over a 100 gallon tank 5x thats a 500gph pump. Or if accounting for efficiency loss 750gph. Thats a ton of water. 
 

I would argue flow rate doesn’t super matter once you get into larger tanks… unless you’re running marine, but then using fans and internal pumps to supplement the flow of the filter is best. Volume in the sump/filter housing is what matters. You want a large stable population of bacteria in larger tanks. 

[Dork Fish]

@Biotope Biologist That is great news. So the "rule of thumb" for 10x in heavily planted tanks doesn't necessarily apply to larger tanks?

For example 6x turnover from an FX4's Max Flow Rate of 450 GPH would be just fine for a heavy bioload/planted tank?

Edited June 22, 2023 by Dork Fish
grammar

[Dork Fish]

To clarify the above is assuming a 75 gallon tank.

[Biotope Biologist]

I don’t know enough about the fx series but id try to see the volume of water the filter itself holds and use that. I know a ton of people use the fx6 for large tanks 150+ and that filter has a ton of capacity. 
 

I have a 20 gallon sump on my 50 gallon. I don’t remember how much water the pump pushes. But I care more about using the sumps volume to provide high quality water to my inhabitants. 

[Dork Fish]

@Biotope Biologist The FX4 has the below dimensions, and volume. Is there a "rule of thumb" for canister volume to tank size, like there is for water turnover?

[AndEEss]

It's not the amount of water the filter can hold, but the volume/surface area of the media it uses. Additionally, from a FILTRATION perspective, flow rate isn't super important. However, from an oxygenation perspective, flow rate is important as it will greatly influence surface agitation. 

My Waterbox 6025 (60.5 x 25.3 x 21.7", 143.8gal) has a FX6 and FX4 running in it. However, those filters/pumps also drive a DIY inline heater, a UV filter and a Cerges reactor. So, while on paper I get about 1000gph (450gph + 563gph) of turnover, I'm guessing it's closer to 900gph with all of the hoses and hard bends in the equipment. Still, with ~120 gallons of water in there, that's way more than enough to satisfy both filtration and oxygenation requirements. 

 

 

Edited June 22, 2023 by AndEEss

[Dork Fish]

@AndEEss Do you run an air pump, or is the flow rate and surface agitation from the filter enough to not need an air pump?

Do you think that one FX4 or an FX6 would be more than enough for a 75 gallon heavily planted tank? Or would you recommend something more like 2x Fluval 407s?

[AndEEss]

I think you'd need just a single FX4 for a 75g tank. Just make sure that you perform monthly maintenance on it (take the foam out, slap it on a rock, return to filter) so the flow rate doesn't drop down with detritus accumulation. 

I don't run an air pump. There's a good amount of surface agitation and the many fish I have in there seem to be doing just fine. 

[Dork Fish]

@Pepere Sounds like a FX4 is enough for a heavily planted tank then 🙂

Would you recommend 1 FX4 with a long spray bar or 2 smaller canisters with shorter spray bars?

Pricing out the difference between 2 HoB filters ($120-150ish) vs the cost of canister filtration ($350ish for 1 FX4 plus spray bar and $400ish for 2 307s).

[Dork Fish]

@Pepere Good to know, I will have to keep my eye out for a used FX4 🙂