Help me understand (and improve) my filter flow - Fluval Flex 15-gal

[Freeway8]

I once had a 35-gallon tank with a canister filter. Tight, sealed, easy to understand the how the water flows, and how to stack the media. Now, 20 years later, I'm setting up a Fluval Flex 15 gallon. I've inserted 2 photos to help with my description of the "stock" setup. They are cutaway views looking from the back of the tank, which has a partitioned area for filtration, pump and heater. You can think of this tank as basically a cube of about 15" in all dimensions.

The first photo shows the 3 sections in the back, and the water flow. Water enters the section on the right from the main tank, thru the slotted intakes both top and bottom. It moves to the left thru slots in the section divider, into the middle, or filter, section (yellow arrows). The stock filter is a snug-fitting block of medium-to-coarse black foam, with cutouts to insert other filtration media. (The included media, just FYI, is a mesh bag of Biomax ceramic rings, and a mesh bag of carbon. But this is NOT about media.) This foam block sits on a shelf in the middle section. From there, the water flows out the bottom of the foam block, and to the left section, which contains the submersed pump and heater, then is pumped back into the main tank near the top blue arrows).

The second photo I doctored to show a more accurate description of the section divider, and how I think the water is actually flowing. I added red arrows to show that there are actually slots in the section divider all the way to the bottom of where the foam block rests. I added an orange assumed water path, knowing that water will generally take the path of least resistance. I think at least some, if not most, of the water flow will bypass the inserted media, and just flow thru the lower corner of the foam block.

So - in my opinion, not a great filtration setup. (I had initially setup this tank with the stock filter setup as shown, and began the fishless cycling process, but stopped that and drained the tank after finding I needed to do a LOT more research before stocking).

I know there are 3rd party suppliers on Etsy and elsewhere that make 3-D printed media basket inserts, some pretty pricey, that claim to deliver superior filtration for the Flex 15. I'd appreciate any input you may have, especially if you are using one of these. I'm not opposed to paying the price if it does the job.

Thanks! And again, this is NOT about filter media, of which there are many options. I think I have a good understanding of that. This is about water flow through the media.

Ed

 

[CarissaT]

I can’t really comment on exactly how it works since I don’t have one, but since there are no fish yet you could set it up with plain water, get it going, then add a drop of food coloring and see where it goes? Maybe? If you’re able to see. Which may not be possible.

one thing I thought of, is that where the water goes is going to depend on how much flow there is. If for instance there was a very slow flow, then water could easily go through the path of least resistance. But as you increase the flow, there’s only so much water that will go through that path without basically failing over to the rest of the area because the flow rate is too high to accommodate a small path. Picture a super fast flow pump that would empty it in seconds, the water would not just run through that tiny area but would blast through every possible hole (well maybe not blast since it’s only gravity feeding it, but it would still go pretty fast).

 

[Freeway8]

I can’t really comment on exactly how it works since I don’t have one, but since there are no fish yet you could set it up with plain water, get it going, then add a drop of food coloring and see where it goes? Maybe? If you’re able to see. Which may not be possible.

one thing I thought of, is that where the water goes is going to depend on how much flow there is. If for instance there was a very slow flow, then water could easily go through the path of least resistance. But as you increase the flow, there’s only so much water that will go through that path without basically failing over to the rest of the area because the flow rate is too high to accommodate a small path. Picture a super fast flow pump that would empty it in seconds, the water would not just run through that tiny area but would blast through every possible hole (well maybe not blast since it’s only gravity feeding it, but it would still go pretty fast).

Thanks for giving this some thought. Your food coloring idea is a good one, I may try that, although the back of the tank has a dark coating on the outside. Somewhat translucent, but not clear like the cutaway photos make it look.

And you're right, if some amount of water takes and fills the "easiest path", additional water being forced (suctioned by the pump) thru will go thru elsewhere. I still think there is a lot of room for much of the flow to not go thru the media pockets, because they're surrounded by the porous foam.

 

[CarissaT]

Yes that appears to be the case, unless there are barriers there that aren’t visible. In this filter I would basically view the foam AS the media. What’s that top one supposed to be anyway, carbon? Edit: never mind I just saw you answered that above.

If you get a lot of water flow through one spot in that foam it will be likely to gum up with dirt first thus blocking some of the flow also.

If it was me I think I would just use one big sponge and be done with it. And if you wanted biomedia you could put some in a mesh bag in the pump compartment.

 

[Freeway8]

What’s that top one supposed to be anyway, carbon?

Yes, it's a mesh bag of carbon. In the bottom cutout is a mesh bag of bio rings. Not an ideal setup at all, IMO, even if you swap the positions. You're right, the foam IS the media, primarily. This is why I'm searching for opinions on alternative media basket inserts, to replace "big foamy" lol.