My experience with MM is that it really likes co2. When I switched from DIY to pressurized, it responded quickly and grew new nodes fast.
I've only had minuta, so I probably can't talk much, but to be honest, I can't tell the difference between them in their submersed state under lower lights. In my 10g under bright light and loads of co2, my minuta was *tiny*. When I moved it to my 29g, which has less intense lights, the leaves grew bigger to compensate. When I moved a few nodes to my mom's very low tech tank, it basically didn't grow many new leaves at all and slowly started to fade out.
My 29g has two of the same light fixtures that you have on yours, with three working lights (I really need to put some new bulbs in...). It has pressurized co2 and I dose EI dry ferts. I have Flourite substrate. I can grow a variety of plants in the tank (including MM to great success), but it needs the co2 and ferts.
I would start dosing ferts, or at least putting in some root tabs. Using Excel probably wouldn't be a bad idea either, or some DIY co2.
The definition of insanity is doing the same thing over and over again and expecting different results.
- Rita Mae Brown