DIY people, please help me.

Why are you doing this in metric? Metric PVC fittings are going to be very hard to find in the US.
Use 2 inch PVC tubing instead of 50 mm PVC tubing for the reactor.
Then it looks like the inner diameter of the fx5 hose is 25 mm.. that's basically an inch.
Go to a hardware store, but a 1" hose barb and see if it fits on your fx-5 tubing (it should, if not, return it)
This whole project can be done in imperial fittings a lot easier..

Unless the fluval hose is far from a standard size you should be able to work this out using imperial sizes. Seems a lot of folks use 1"ID hose on their FX5s so it shouldn't be too difficult

Something to consider, I just built one similar for my tank. I went with a 3" pipe for the reactor. Going larger will slow the flow through the pipe and give more contact time with the CO2. My pipe is about 12" long or so (stupid short stand) but works very well. I can take some pics tonight for you if you like.

I'll get some for you tonight.

I will warn you, I did mine a little different. I wanted to fit everything into my very small stand, so I wanted the reactor to stand on its own. To give you an idea, I'll like the parts I used. Starting at the top I used one of these guys (so it could be right under the bottom of the tank with no kinks in the tube). I lost some flow from the 90 bend, but it still flows well-

http://smile.amazon.com/gp/product/B00176QMQW/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1


From there, I went with a bushing, 3/4" threaded up to 2" slip-

http://www.mscdirect.com/product/de...gle-PLA+-+Test&CS_003=7867724&CS_010=36999779


I went with the above because I couldn't find a 3/4 to 3" bushing, so you need a 2" to 3" adapter-

http://www.zoro.com/i/G1687427/?utm...hopping_Feed&gclid=CMPiqdXwpcQCFZI1aQodDJYARw


Next is just some basic 3" PVC. I dont think I need to link that one

Following that, I got a little crazy and added one of these-

http://www.lowes.com/pd_23428-1814-..._product_qty_sales_dollar|1&page=2&facetInfo=


Little different than most people, but it is working well. Sure, some flow goes past and gets trapped, I'm sure some crap is stuck down at the bottom but I'm not overly worried about it. It makes plant food lol.

Continuing down towards the bases, just a bit more PVC then a basic 3" cap. I went with a rounded one because I couldn't find a flat one. Propped up it works.

Now going out the narrow side, you need another one of the bushings, same as before-

http://www.mscdirect.com/product/de...gle-PLA+-+Test&CS_003=7867724&CS_010=36999779


From there put one of these on the end-

http://smile.amazon.com/gp/product/B008FML2NY/ref=oh_aui_detailpage_o06_s01?ie=UTF8&psc=1

And you should have it! Not pretty or streamlined but it is working really well for me. I'm doing more bubbles per second than I can come close to counting and it dissolves them all. I wish it were a bit taller but it works.

All of the parts besides the barbs I was able to find at Lowe's, I just couldn't find them on the website. I did a test fit in the store to make sure it all fit together.

A word of advice, use a lot of PVC glue, more than you think you need. First one I built I didn't and I got leaks, nothing terrible but enough to make a mess. With the way it is glued there isn't a way to fix it without cutting it apart or remaking one.

Also, don't forget to drill your hole for the CO2 tube and pull it through before gluing everything!

On the link I gave you, the picture has the piece upside down from how I used it. As you said, I wanted something I could stand up. You could just use a "T", but I wanted something sloping to restrict flow a bit less. Also this narrowed down the pipe so I'd need less pieces. The part sticking off is only 2" so I only needed the bushing rather than the piece to size down the pipe too.

But then yes, just a cap down at the bottom. Give me a couple and I can try and draw a picture for you

Its rough, but here is the basic design. Sorry, ran out of room to finish the second barb on the left, but I'd like to think its easy to imagine. Any threaded fittings dont forget to tape.

Also, I'd skip the hose barb for the CO2 line. Drill an undersize hole and pull the tube through it. It'll cost less and it is less parts to leak. You also want CO2 in the center of the water, not the edge.

And you do have extra room, I only have about 18". I'm jealous! lol