Buy a bunch of similar sized pieces of slate. Put them in a seperate, heavily lighted aquarium full of used aquarium water (removed from your display tank during water changes). When the algael growth is heavy, put the rocks in your display tank, to be grazed on. Just keep rotating the rocks in and out.
You can keep other fish in the "garden" tank, as long as there aren't any algael eating fish. I used to keep large predators in the tank, and also would use the predator to eat excess/unwanted fry.
I'm probably not describing the process well, but it's easy to do, and the mbuna to appreciate it.
From my understanding and reading, it seems like the net is there to hold down the food and make the fish kinda work for thier food.
...About the "garden tank"...do Af. Cichlids really like algae that much? and wont that dirty up the display tank too much? I am kinda new at this so excuse my ignorance!
Africans do like algae, but they won't keep your tank clean. Sounds like the garden tank is supposed to be messy and the show tank would just have a nice green rock added on occasion and removed when the algae was gone.
DJRansome is right. The "garden" tank is messy, the display tank is clean.
As long as you choose the garden tank's fish carefully, they won't prey on any invertebrates. There will be some, though not the exact species that come from Lake Malawi. I'm not sure how they get in there, but they do seem to.
When I did it, I used the garden tank as a "utility" tank...the fish were large predatory species that disposed of unwanted fry. The algae rocks were just a side benefit. Keep in mind that at the time, I had 19 other fish tanks...about 10 of those contained actively breeding cichlids. The local fish stores would only take a limited amount of fry.