Could the MC be melting because of too many nutrients? Because I dose EI dry ferts and have really deep Ada AS so the plants would be swimming in nutrients.Great to see the update
Re MC melt I'd look to water column fertilizing (as your CO2 & light seem fine) - MC has more root structure than HC but I still suspect it needs suitable water column "food" if your growth is relatively fast
(I'm rather good at slow small growth MC - never observe transparent leafs)
How big are your "gold" tetras now? still gold?
There are likely a couple species that ship as "gold" or "brass" tetras (all wild caught if displaying the "gold") - one species seems to remain considerably smaller ... last shipment at my lfs appeared to have both species
Kessil fan here too
The tetras have remained the same size but have become even golder than before. Whether that's down to food or just the light I don't know. But they're quite golden.
And I'm kicking myself for not getting the kessil from day one ( which I wanted to do).