I have a Fluval Flex 123L AIO tank with limited cabinet space. I'm looking into adding a canister filter for better filtration.

[Bmr4Reef]

So I've already upgraded to 3D printed filter boxes running 3 stages on both sides as well as the largest Sicce pump that would fit in the stock location. Been that way for over 3 years. I'm still stuck with high nitrates and phosphates even though I don't over feed. The only thing that makes the tank happy are water changes.

There are 8 fish in the tank.

2 clowns
blue tang
diamond watchman goby
tailspot blenny
six-line wrasse
some other tiny guy


For canisters I'm looking at Oase and Fluval FX. I only have 15x15x24" tall in my cabinet so sumps are out. Any Oase should fit but for Fluval I may be stuck with an FX2 width wise.

If the AIO running as a prefilter still fails and I go FX I would then add an inline prefilter to cut down on canister cleaning.

 

[Fish Fan]

Congrats on a nice setup! I didn't know these Fluval Flex tank came so big!

Sorry about the late reply, I thought I posted this like an hour ago lol!

Just some quick thoughts, I'm sure others will be along to offer even better advice shortly, but......

I am strongly thinking that you have too many fish in your tank, like way too many.

I have never had one, but I know that the Fluval AIO's typically come with some huge, huge sponges and ceramic bio media that's really a throwback to freshwater. The sponges in particular will trap detritus and could potentially cause high nitrates.

In my very humble opinion, adding a canister filter is not the answer here for a few reasons. The poster you replied to in another thread had an atypical design that I just don't believe would work well at all.

Many here run the Fluval Flex tanks, I'm sure you can too

I'd suggest ditching the big, black sponges, and about 90% of the ceramic bio media that the tank came with (along as just about any other media the tank came with).

You want your rock and sand to be the main Biofilter in your tank. The 10% biomedia I suggest that you leave in your tank would be for an emergency QT tank setup, which you're likely to need given your choice of fish in your small tank. Just saying

I'd run some filter floss or Poly-Fil (same thing, just cheaper; found at Walmart and craft stores) and swap it out at least weekly if not sooner.

Additional space for media in your AIO *can* be used for other media like carbon, but that's not going to help your problem.

I really don't typically say this, but you have too many fish, and fish that don't belong in your tank, that's why you're nitrates are high. Consider rehoming some fish, or know that you'll have to pull and replace your mechanical media (filter floss) as often as possible, and do large, regular water changes to keep things in line.

I'd rather see you get a quality hang on back skimmer or some kind of refugium before hooking up a canister filter to your tank.

I hope that helps! Good luck!