What about the difference in masses of these species & bioload produced by them? We are now upping the minimum tank size for the fahaka at around 180g.
Bioload can easily be managed via water changes & proper filtration.
This is precisely in line with the old Army Corps of Engineers dictum of "The Solution to Pollution is Dilution" - which is found in your sig line, Pufferpunk.
The reality is someone with a 500 gallon system and a dialed up auto water change system could potentially provide much higher water quality, and a much higher quality of life, than someone who has a 1,000 gallon system and isn't as attentive with their water changes, or filtration.
Personally I would much rather see an adult Mbu in an 8ft x 3ft x 3ft" tank (approx. 540 gallons), with consistent high water quality, vs one kept in a 1,000 tank with only average water quality.
I think that in this hobby there's a fine line between being compassionate, and being fanatical.