Why are none of the major power filter manufacturers producing a
hang-on-the-back power filter designed specifically for small planted
tanks?

Design Criteria:

-Power Filter/CO2 reaction chamber combined.

-Integral connection for a CO2 delivery tube. Like a venturi tube
connection...pre-impeller would be best.

-Air tight sealed design beyond the CO2 injection point. No CO2
leakage.

-Long water return path with baffles for maximum CO2 absorbtion time.
This could alternatively be a chamber with a dual use
biofiltration/CO2 bubble chopping type media.

-Below the surface water return to minimize surface turbulence.

I suppose that something along the lines of a HOT Magnum ( if it had
the connection noted above ) would be similar to what Im
thinking....but...I want something that is the size of an Aquaclear
150 for my 10-20 gallon plant tanks.

OR..... maybe something modeled on a paradigm like the Prizm or CPR
Bakpak small protein skimmers for marine tanks...without the protein
skimming but with filtration and reaction chambers.

As much discussion as there has been on effective means of creating an
efficient CO2 reactor for small tanks.....seems like an idea that one
of the manufacturers would have explored.

If I was in any way skilled with acrylic....I'd try my hand at
building a prototype.