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jgray152 03-21-2008 08:58 AM

Fluval Fx5 Modification
So as we all know the Fx5's design is not the very best for media capacity. I have been thinking of ways to improve this and some ways have been modifying the baskets but this would be way to much work and would still yield little increase in media capacity.

So I though, why use the baskets? They are only there to seporate the media and for ease of cleaning for the standard consumer.

The modification that I have done should increase the bio capacity well beyond the Ehiems 12 liters. It could be around 15 liters. I have more testing and figuring to do but here is a picture of what I have done so far. I have only cut 2" thick open cell foam to fit snug for zero bypass so far.

I removed the intake tube that is inserted on the cover so the water just falls right onto the foam.

Im going to use egg crate at the bottom to have a safe gap between the impeller intake and the bio media.

Im going to be using Bio Bale is my bio media instead of the Fluval Bio Max which I have had great success with. The bio max would be way to expensive for the amount I need compaired to the bio bale which has TONS of surface area.

This modification is good for a few things.

1. ) Increase bio media capacity. Possibly around 12-15L
2. ) Decrease velocity of water flow for more contact time with bio media
3. ) Large area for flow which decreases the flow restriction.
4. ) Should help increase flow output since the water only has to change direction ONCE instead of THREE times.
5. ) Cleaning should be even easier and you will not need to remove bio media to clean the spunges like you had to before.
6. ) Eliminate about 5 possible warn o-ring issues
7. ) Original Foam blocks don't seal as nice as I would have liked to the basket above.
8. ) Increase the time between change intervals. In original form if you had fine filter foam in the center baskets, they could clog up real quick causing a drastic decrease in flow and lots of cavitation of the impeller and you would get LOTS of air bubbles everywhere. Should be able to use fine filter pads for longer periods of time.
9. ) Due to the decrease in flow resistance, hopfully this will help with the cavitation problem in the Fx5.

I may think of more reasons but so far this seems REALLY good.

What ya think?

jgray152 03-23-2008 09:53 AM

I just did some compairing between the Fluval 405 and the Fx5.

405 Media Capacity: 4.2 Liters
Fx5 Media Capacity: 5.9 Liters

Difference............... 1.7 Liters :oops: :blink:

For a Filter so LARGE they only managed to put in another 1.7L of media. :rolleyes: :shock:

The Little amount of media with such a high flow rate, no wonder it doesn't cycle very quickly. The Flow velocity is to great for such little media capacity.

Mechanical Area;
405 Mech Capacity: 49 sq In
Fx5 Mech capacity : 325 sq In

276 sq In increase in Mechanical Filtration from the 405 to Fx5. YIKES! They must have been looking at time between cleaning intervals

Rough estimates for new Mechanical and Media Capacity. Need to take more measurments before I find the right figures.

Modded Fx5 Media Capacity: 41 liters.
Modded Fx5 Mech Capacity: 314 sq In

jgray152 03-23-2008 11:08 AM

Rough estimates for new Mechanical and Media Capacity. Need to take more measurments before I find the right figures.
Modded Fx5 Media Capacity: 41 liters.
Modded Fx5 Mech Capacity: 314 sq In
Measured Unmodded Fx5 Flow: 525 GPH

Will the flow increase even more? We'll have to see. With a pump output of 925 GPH, there is some room to grow. The said filter output by Hagen says 607 GPH. Its actuall output is 86% of the said output which is pretty darn good but in comparison with the pump output thats only 56%. Where is the restriction? Thats what we need to know and I want to find out. With almost 50% restriction that can easily put the impeller in the braking point of cavitation even with clean media.

I have a feeling the baskets are a small part of the restriction and its more of the fact that the intake of the impeller is smaller than the output. Flow meters will be needed to measure flow increase.

Eheim 2080 Media Capacity: 12 liters
Eheim 2080 Estimated Mech Capacty: 144 sq In. Est. 2.3 liters
Measured Unmodded 2080 Flow: 318 GPH
318 is 70% of the 450 GPH pump output which is really good.

jgray152 03-28-2008 08:25 PM

So I took some measurments and did some math.

I have an area of 11.50" in Diameter and 8" High for bio media. So 11.5 / 2 = 5.75 x 5.75 x 3.14 = 103.81 x 8 = 830.5 cubic inches = 13.60 Liters of bio media. I could raise that to 14 liters very easy by just raising the spunges another half inch. :)

5.3 Liters of Foam which is about 325 cubic inches.

So what do you think? My first estimations were before I took some real measurments.

Here are some pictures of the modified Fx5. All I need now is the media that is on route

Picture of some of the egg crate cutt with my band saw and siliconed to the side of the basin to aid in support for the top rack.

Same as above except with the bottom rack in place.

Now a picture of the top rack in along with the bottom rack

Just a close up

jgray152 03-30-2008 06:09 AM

Here are some updated figures;

Unmodified Fluval Fx5 Stats
Media Capacity: 5.9 Liters (im going to measure this since I think its wrong too..)
Mech capacity: 294 sq in; 369 cu in; 6 Liters
Measured Flow Rate: 525 GPH
Max Head Height: 3.3m (10.8ft)
Filtration Volume: 11.2 (Hagen says 20L. I call BS)

Modified Fluval Fx5 Stats.
Media Capacity: 14 liters.
Mech Capacity: Est 108 sq in surface area. 325 cubic inches; 5.3 L (2" prefilter foam and 1" fine filter foam.)
Filtration Volume: 19.3L
Measured Flow Rate: ??

Ehiem Statistics
Eheim 2080 Media Capacity: 12 liters
Eheim 2080 Estimated Mech Capacty: 1.5L; 92 cu in
Measured Unmodded 2080 Flow: 318 GPH
Max Head Height: 2.2m (6.6ft)
Unmodded Filtration Volume: 13.5 L

jgray152 04-05-2008 06:10 AM

I had to rethink my design a little bit so that the purging could still work. I needed to make the intake submerged in water at all times so air could not escape up into the intake.

So. This is my idea. basically, im wanting to keep the intake submerged in water when the filter shuts off. I think I should have made the pocket a little deaper than it is.

I had to lower the top tray to keep an inch of gap at the top surface so water can flow. Otherwise the top cover will seal against the spunge. Now the filter holds about 11.9 liters of bio media. I could cut the 2" spunge in half and get back up to 13.7 liters but I think it will work just fine.

As large as the Fx5 looks, the inside doesn't have much space to work with.

jgray152 04-05-2008 06:11 AM


I hooked the filter up and did a test run.

It seemed to have worked pretty good. The system had purged a lot of the air out through the output on its own. Flow seems to be the same as before.

One thing I have to look into. The canister will not purge 100%. Air is still trapped inside all the time for some reason. Even if I shake the filter and tilt it about 50 degrees to the side no air comes out.

I removed the lid and started looking at the filter foam. I noticed there was a bunch of small air bubbles forced into the coarse foam right where the jet of water comes from the small air purge tube.

So I thought that maybe the air was getting trapped in the media. I started lifting up the coarse foam and noticed no air trapped under it. I than lifted up the fine filter pad and noticed a few small bubbles surfaced. I also then lifted the very fine pad and one good sized air bubble surfaced.

I removed the fine filter pads and kept the coarse pad in and im still having a problem with air.

Im going to try without any foam to see what happends.

I havn't checked but possibly the tube is getting clogged......

Im really going to look into this as far as my knowledge will let me.

jgray152 04-05-2008 06:11 AM


Within an hour all the air has been expelled. Most likely worked out by the impeller. There is no longer air in the canister. This is only with the coarse spunge and I would imagine that with the fine spunges the air would not be able to get worked out as easily.

Flow is right up there. I think its a bit more than before but this could heavily be due to the dirty filter fiber reducing the flow in the Fx5 I was running before.

Its been running overnight without any issues so far and is as quiet as it was before.

Overall im very please with how this turned out. There is just a small bug I have to squish in the new design.

I think it might be really neat to cut the center portion of the cover out and silicone or hot glue or PVC glue some clear plexy glass/lexan over the top so I can see what is happening :)

My Bio Bale has come in so im going to be putting that in later on today since im at work right now.

While switching out filters and allowing the modded filter purge it loosend up a lot of junk in the hose which was thrown everywhere in the tank. The filter picked it all up and held it in either in or on the coarse spunge. Before I removed the fine spunges they too seemed to pick up a lot as well.

So far the general function of the filter seem to be exactly what I wanted.

I may try to increase the inner diameter of the air purge tube but I would rather not have to do this since that would decrease the output further. It may not be anything I would be able ot measure though so I may be alright. Increasing the ID of the tube I would think may help with purging the rest of the air. Thats as long as the air is reaching the tube.

I know installing a bulkhead at the top of the canister will solve this issue but that seems like a quick hack instead of a fixing a design flaw with the moded filter. So with that said, the tube should do its job.

jgray152 04-07-2008 08:03 PM

There seems to be some confusion as to how the water flows in the Fx5 after its modified so I did a quick sketch and here it is.

jgray152 04-08-2008 09:38 PM

The filter now purges 100%!!

YES! My cover modification worked!!!

Here is a pic of it.

What you see is plexi glass that was heat molded so that is concaved, high on the outside and low on the inside where the 3" PVC fitting is. Then silicone all over the place which I may have to redo since I forgot to take off the protective plastic covering on one piece of the plexi and its on the underside of the plexi so the silicone may seporate. If so I will just fix it.


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