Sorry Sam, but I can`t be bothered to read all of this thread.
Going back to the beginning, how did you go about planting the HC? If it was emersed growth in the pots, what I do is leave them in the pots underwater with good CO2, ferts etc for two or three weeks. Then I plant them out individually, almost burying the entire plantlet. I have great results with this method, but the HC does take a little while to kick off. I now find it a bit of a nuisance because it keeps popping up out of nowhere and outcompeting my UG.
I think the shock of suddenly becoming immersed, plus being planted out can be a bit too much for it, especially in a new tank where the CO2 levels haven`t yet been sorted.
I have just started a 120cm 2.5 weeks ago and am using an Aquamas CO2 reactor, with 2 Tetratec 1200s blowing in from either side. Flow distribution around the tank seems pretty good so far. Your internal looks a monstrosity to me.
By the way, I can`t help but think you have a very low plant mass. In my 120cm, to stave early algae problems I have carried out the following steps:
Used mature filter media.
High CO2 from the word go.
Short photo period that will slowly be extended in time and intensity.
50% water changes every day for the first two weeks, gradually decreasing to once a week over the next month.
Zeolite and Purigen in the filter.
Fast growing stems planted, that may make way later on for Crypts etc.
Make sure you hammer the water changes, Sam.
Dave.