• You are viewing the forum as a Guest, please login (you can use your Facebook, Twitter, Google or Microsoft account to login) or register using this link: Log in or Sign Up
  • You can now follow UKAPS on Instagram.

Mark's Fluval Flex 123L


10 Sep 2021
First tank in a long long time with the following specs

Fluval Flex 123 vertical bowfront tank on a custom drawers/filing cabinet stand
Inbuilt fluval aquasky 2.0 lights (soon to add the aquasky plant 3.0 for extra light)
I am going to try out the Colombo CO2 rector which uses baking soda and citric acid to pressurise a cylinder.
18L Tropical aquarium soil topped with powder for substrate
Siryu stone and Ent wood from aquarium gardens as my hardscape

I have just started a fishless "Dark Start" cycle by adding some pure ammonia to the tank (3ppm) and keeping the lights off. In a few weeks time, once the cycle is complete i will aqua-scape and add fish.

Would love to hear views on this hardscape as I still have ample time to change it before planting.

Today was planting day after nearly 3 weeks of dark starting. Decided that I did not like the hardscape at all so i re-did the whole lot and i think the whole thing feels a lot more balanced now. Pretty sure i'll be fiddling with the big piece of wood cos im not quite 100% happy with that, but on the whole i think it's much more easy on the eye.

I also ordered far too many plants!!

Rotala Orange Juice
Rotala rotundifolia H'ra
Rotala rotundifolia Green
Cryptocoryne undulatus Red
Myriophyllm Guyana
Hygrophilla Iancea Araguala
Alternanthera reineckii Mini
Staurogyne repens
Monte Carlo

Last edited:
Sorry - I'm not quite grasping this concept. I think what you're saying is your tank isn't big enough, but you've phrased it oddly.
Yeah, my bad. It's been 15 years since I ran a tank so I'm pretty rusty. I guess i'll just have to go and tell the wife I need another tank. No-one likes to waste plants.
1 week after initial planting update. Pretty good growth across the whole tank. Monte Carlo has started spreading out a bit and the stem plants have really taken off as well.

Added fish too,

Harlequin Rasboro
Cardinal Tetra
Longfin Zebra Danios
Amano Shrimp

Picking up a few ottos on Friday.

The Colombo Co2 reactor is performing better than i thought. I estimate it should last 1 month on one refill of citric acid and baking soda running about about 1.5 drops per second. Still plan to upgrade to fire extinguisher in the near future i think.

I removed the noisy and downright awful pump that came with the tank and replaced it with 2x eheim compacton 600s running in parallel. That gives me about 10x turnover.

Also cut out a gap in the lid and installed a USB PC case fan to help remove some of the heat. The Fluval LED lights get suprisingly warm and the tank was getting quite warm towards the end of the day. I'm now at a solid 24 degrees all day long.

2 week tank pic update will come tomorrow, but this week I've added my solenoid and a temp probe to my home automation system so that I can get alarms on my phone if the temperature goes out of range. Next job is to see if i can reverse engineer the Fluval bluetooth to get a status of the lights on my dashboard too.

Since i added the fan, the temp has been a stable 23.7 - 23.9.

Think i have lost 1 Amano shrimp as i've only seen 4 at once recently.

Ordered the co2art dual stage regulator and a couple of fire extinguishers today as well so looking forward to setting that up as a replacement for the colombo reactor which i'll probably end up running on a new nano tank that i plan to set up.

Screenshot 2021-10-13 at 18.12.24.png
2 week update

Moved the lower bit of wood slightly so it seems to flow better.

Generally good growth, especially the monte carlo front and center. Got good coverage there already. Java moss seems to have a small bit of fuzzy algae in it but is still growing pretty well.

Thinking or moving the hygrophila araguaia more to the front as it's hidden away a bit right now behind that central piece of moss covered wood.

3 week update...

Upgraded CO2 to co2art regulator and fire extinguisher as well as making my co2 inline. I am in the middle of the "new tank diatom bloom" it seems, with the monte carlo getting a bit brown as well as on the rocks and wood. Wood is also getting a bit green. Have dialled the lighting intensity down for most of the day with just a small peak in the middle of the day (i have the Fluval Plant 3 so am following Bentley Pascoe's "daylight sim" methodology. Generally happy so far but will be much more satisfied when the brown goes away. Added a few more plants in some gaps and also added a handful of nerite snails to help with the algae.

After having some issues with cyanobacteria gathering on my monte carlo and other plants, I did a 3 day blackout and reduced my light intensity down to 60% thereafter. The monte carlo looks a lot cleaner now and hopefully the light reduction will help prevent any future occurrences. I had my 2 fluval lights at 100% for a time - i think that was a stupid thing to do at this early stage of the tank. Pretty sure that CO2 flow is good (both outlets point down towards the front bottom of the tank and i can see co2 bubbles throughout the tank). I'll post an updated pic towards the end of the week.
I am scanning this forum for experiences with the Fluval 3.0 Plant LED -- since you use one: what are your thoughts on it? Happy with it, or not impressed?
Im really happy with it, but then it's my first venture into LEDs having been away from the hobby for 15 years. I love that you can adjust the colour balance and the pro mode stuff is great, allowing you to set up to 10 settings over the course of the day.

As for Intensity, i'm still on the fence. On one hand i see people saying that the "fluval lights arent very powerful" but then i also read that at 16 inches its about 150 PAR on max power which is plenty for all plants. On the whole, im very pleased with it. The only real issue I have with it is that it doesnt store the settings on the light itself, so in the event of a power cut you need to connect the app to it to send the settings over again. Similar issue with change of clocks; when the clocks went back at the weekend i had to connect the app on sunday morning to update the time on the light.

On full whack it does get a bit warm too, enough to heat the water in my flex123. Bear in mind that its directly above the water surface in my setup with very little air flow. Adding a pc fan that comes on at a certain temp has solved that issue however. In a open top tank this wouldn't be a problem

Bentley Pascoe on youtube does a good series on how to set the whole thing up - i'm using a shorter version of his daylight sim profile and have no complaints other than algae issues that i have caused myself.
Thanks for your reply! I've seen the videos of Bentley, very useful for understanding all features of the light and setting for different sizes. I don't have a big Flex like you, now cycling a Flex 57L. The last scape I did with some hard plants (who need a lot of light) definitely suffer with the standard lighting of the Flex. This time I stumbled upon the fluvial 3.0 which gives me the flexibility on scaling down the light (for longtime use), but also can ramp up the intensity when needed.

You say its getting a bit warm because you attached it to the lid somehow? I was planning the exact same thing, so mounting the light directly on the lid.. as I am living in The Netherlands I don't think extreme warmup of the tank will occur, or am I wrong?
I find that the LEDs actually get a bit warm and when running at 100% on all colours and being enclosed under the lid with the included Aquasky light (you can fit 2 fluval lights in the 123), my tank temp was rising by a degree or two during the day. I'm in the UK, so pretty much same weather conditions and temperature - the room temp was less than the water temp. If you are just running it alone on the flex 57 you will probably be ok. The flex 123 has a thing you can cutout which fits a 90mm fan perfectly. Not sure if the 57 has the same but it works really well for me
So I guess I'm about 5 weeks into this tank post planting and things are going pretty well. Growth is good, but unfortunately I'm also growing a bit of cyanobacteria. I'm putting this down to increasing my lighting which drained all my nitrates. That coupled with low oxygenation (now sorted with the skimmer to remove the surface film I was getting and the venturi to increase surface agitation), we seem to be heading into a better place. I'm also dosing Easy Life Blue Exit at double the recommended dose just to help with killing it off once and for all. I'm three days into the 5 day course on that. A 3 day blackout just about worked but there was some that came back. I also lost some newish plants and i dont really want to go through another blackout so soon so am trying this Blue Exit stuff. I've heard mixed reviews.

Did a little bit of moving of plants around (swapped the Hygrophila Polysperma and Rotala Orange Juice so that the HP has less light and the ROJ has more) and added some Pogostemon Erectus to fill out the dead center of the tank.

I've got my lights running on 25% at the moment for 6 hours to try to help kill off the cyano but fully intend to start ramping that back up once its all cleared off.

6 Weeks In

Had to do another blackout to get rid of the pesky cyanobacteria. That, along with the Blue Exit seems to have done the job for now. Now that I think i've worked out what caused it, I am hoping it wont come back. Have increased my nitrate dosing by 25% and added extra oxygenation at night with the venturi into the skimmer.

The only think frustrating me is that bits of the monte carlo carpet keep coming out, especially in the corners of the tank. Then it's a bit of a pain to get it planted back in without it floating off. The center section is solid however, and i've given it it's first trim.


I've also started a grindal worm culture which i hope to start harvesting in a few weeks.

2.5 months in and pretty much all the algae has now subsided. Just a bit of diatoms hanging around on some of the slower growing plants. Pretty happy with progress but would like some more growth in the corners of the tank.... had to remove the monte carlo there as it just wasnt getting the light, and with my light intensity dropped, they were just dying off. Moved some S.Repens cuttings down there so will see how they go.
So at the start of reading this I was like damn, I would have stuck with the original scape, but great job with this bud. Looking amazing. Did I read that you changed the scape from the original because of better flow this way? Plants look like they're thriving :thumbup:
I just didnt really like the initial scape. Didn't leave much room for planting towards the rear.

As for flow, ever since i replaced the Fluval dual duckbills with my custom spraybar, the plants have gone crazy. Proof in itself that flow is king.