Are you sure its not that the plants aren't receiving enough light and nutrients is why there dwindling or are they shredding the plants to pieces..
I believe I have plenty of light- I watch the AC's shredding them all of the time-I throw all of my clippings from my 29g(java moss/anacharis/hornwort) in with the mbuna so they can have some extra greens to eat...It's strange, ever since the females started mothbrooding, the AC's started to shred the plants...