Probaly the size involved and the DIY aspect.
🙂 They aren't realy for sale in the regular lfs and if so they cost a fair amount of cash. Some can build 'm for you but this also will require some cash flow taking today wages into account. But if you are handy and into DIY you can build whatever you like in any size you need. And it aint so difficult at all.. It just needs some thinking and tweaking to make it all run smoothly. Bt once it does you only will have benefits. The co2 loss is rather not such an issue as far as i know from the people wo are using a sump with a high tech setup. Contact Jason if you have questions about that.. But seeing this might tell you enough..
http://glassboxgardeners.com/threads/jasons-120x60x45-tank-setup-modified-eareef-1200s.473/
🙂 Jason is also UKAPS member and not the only one, i know a few dutch guys running sumps on high tech and they all say, not realy a waste, a bit more ofcourse but not a dent in your wallet. Not sure i believe ukaps member Yohan is running also a high tech sump..
I'm running a 110 liter low tech (The Stove) on a little 25 liter (tank) trickle sump with 12,5 liter water capacity.. It runs like a charm since november last year.. And wouldn't want to have it any different. And it didn't realy cost me that much. €15 for the tank, €15 for the acrylic to make the dividers and €30 for the transparent pvc tubing to make 2 overflow syphons.
This is my setup, didn't close the stand yet, because of tweaking the sump and having free space of work.. But since it is running perfectly the last 4 months i'm done tweaking and testing and about to finnish the stand.. It's a 5 chamber sump also with moving bed chamber. Could have made it 50% smaller and stll would be sufficient..
Why are they not used much?? The only thing i can think off, honnestly?
😳🙄 Hope you don't mind? A lot of ignorance developed on fairy tales going around to excuse the lak of trust or lak of knowledge in experience to build one yourself.. As you can see above space aint realy that much an issue, i wouldn't know what to do with all the space if there was a small cannister standing instead..
😉 Still plenty of room to convert the whole thing to high tech if i wanted to.