Yes you could always go inline. All methods (inline, intank, reactor) work it's just preference, but there shouldn't be any reason not to have the intank one work especially on a 2-foot tank. For now if you just move the diffuser to the side wall so it ends up in front of the return it should work. The filter should have enough to disrupt the bubbles from the other side if you decide to try that. My tank is 3 feet wide and my Eheim 2213 reaches the other side, not strongly but enough to break up the stream.
I did have a power head in my tank for a while to break up the bubbles, but took it out and I haven't seen any difference.
Well I moved things around and yeah it seems to be enough flow with the diffusor moved to the other side of the tank, however I didn't try it today, I would rather the diffusor be closer to the back wall for visual reasons! I think I will actually be replacing it for an in-line.... seems like a better solution to me for now!
In terms of cave loving bottom fish I would recommend loaches. Bearing in mind they are happier in small groups. Fish like the Yo-Yo loach for eg would love those caves. Having somewhere they know can they can retreat to actually means you see them out more often. They are also fascinating fish. They remind me of dogs always sniffing about.
I have dwarf chain loaches at the moment (Botia sidthimunki) and they are fab in my opinion. Though in your setup they will likely end up in your tubing. When mine were smaller at least one was usually in the fluval outlet which has a 180 bend followed by a 90 degree. They would go in there and 'clean' the inside for me.
But the larger ones like Yo-Yo's and their relatives would be too big for that. BTW they are so called because the markings on their sides looks like Yo Yo. Botia lohachata in formality.
My dwarfs live under the foot of an old wooden arch (other end eaten by the ancistrus). It's concave and has at least two entrances. The ancistrus kept trying to take it over by burrowing under it but she is too big and it would just fall in. I got sick of continually having to rescape because of it. So I took the remains of the other end at stuck it in the gravel to give them a wooden floor. Problem fixed.
Hey! I looked up those dwarf loaches, pretty cool fish - though I'm not so sure if I want to invest in them (really quite expensive). Plus I think I have decided to look at this tank like a "big" nano tank. Basically I am worried they might look a bit too big for the scale of everything in the tank. Though I might have to keep them in mind, might have to look up some videos of them on youtube!
I had 3 peacock gudgeons that also seemed to like to explore "cave" structures. I had a coconut shell one would go in and out of frequently. There was also a piece of driftwood that had a small arch shaped bottom. The other 2 were in and out of there a lot.
I kept a pair of those fish in my old 35 gallon tank but to be honest I wasn't a huge fan of that fish, maybe they look better really well fed but the colors with that form didn't suite me. But you're very right, a goby like fish would be a great idea but I don't think I will go with one.
So here is another boring co2 video but this shows the new placement with a bit more current (I cleaned out some muck my filter collected, especially in the first tank it was on for 2 weeks that was a kind of dirty plant quarantine / temporary storage tank thing) - also some photos of the plants getting some co2 bubbles! Finally a photo of the tester finally turned a shade of green!
The side of the tank with the diffusor is certainly getting much more bubble build up, I am very curious what this all means for co2 levels throughout the whole tank albeit it quite small tank... maybe the inline diffusor will help this a bit? I am also still considering throwing in another pump/powerhead thing to help circulation a bit... maybe!
Thanks again for all the input everyone!