Hi everyone, wow, can't believe this thread is STILL going! It's great!
Honestly, how tall is the 75? It might be too talk to penetrate far enough, however without co2 you dont want too much lighting. Light drives the growth speed, all all other nutrients including co2 much be readily available I meet the plants growth speed. If you planning on growing low light plants slow, this may still work!
I just added a fourth done above my 40b, getting ready to step it up a bit since I'm tired of plain rotala and jungle Val's!
40B High Tech Tank - 10g Shrimp and Moss Tank