I'm fairly new to the egg tumbling scene, but I have made a few observations and can honestly say that if you want to save as many fry as possible from the spawn of a mouth brooder, this is the way to do it. My first attempt at tumbling was a food strainer over a current of bubbles. This did not work for me, although I have since seen other hobbyists use this method with success. I guess it must take a little tinkering. I have also seen some very good plans for tumblers that would probably work well, but to this point I haven't attempted to construct one.
My anger toward the Labidochromis lividus is what sparked my need for a tumbler. These beautiful fish were spawning for me constantly but the females would always abort the clutch soon afterward. It became apparent to me that if I was ever going to get fry from this species, I would have to find a tumbler that worked.
To my good fortune, my best friend was aware of my woes and sent me a tumbler constructed by a member of her aquarium club. Like a kid at Christmas, I rigged the gizmo up, and waited patiently for the next spawn. A week later I had brood of Haplochromis sp. "thick skins". Even though the female of this species is a reliable carrier, I wanted to see the tumbler in action, so I stripped her.
Stripping a female mouth brooder of her eggs is quite simple and many people have developed some interesting ways that work well for them, one of the oddest being the use of a turkey baster to push water through the gills. My method is somewhat more primitive but works for me. I have a small tub of water under a bright light ready, then catch the fish in a 12" net. When I am lucky I can grasp the fish through the net in my hand and hang her upside down in the air above the water. Usually within one minute she will spit her eggs out. Sometimes, a fish can be a little stubborn and take some coaxing. Depending on the species and dentition, something simple that often works: I put the fine mesh of the net over the fish's snout, and when she opens her mouth a tooth will get caught in it. A gentle pull downward on the mesh of the net will open the female's mouth, and she will release her eggs.
Where was I? Oh yeah... now I had a dozen "thick skin" eggs in the egg chamber of my new tumbler. I had the air gently moving the eggs around, and things seemed to be going well. I was wrong. Not all the eggs were fertile in this clutch. A couple of the eggs fungused and it spread to the "good" ones. For the first couple of days you must watch for this. Remove any eggs that turn bad right away. After a couple days this is no longer a problem and all you will have left are fertile eggs that will develop and not fungus. I was left with five eggs, and after I cleaned the egg chamber of the tumbler, back in they went and they did very well. The first thing you can see is the vertebrae in the egg. The tail is the first extremity to "pop" out, followed the next day by the head. The new fry tumble gently while absorbing their yolk sacs. When the fry have almost become free swimming wigglers, I take them out and put them in a tank or their own.
The tumbling period is affected, more than anything else, by temperature. The higher the temperature, the quicker the eggs hatch. The temperature also affects the speed in which fungus will destroy the whole brood. I suggest a temperature of about 78 F for the first couple of days of tumbling, then a slow increase to 82 F.
Some species, I have noticed, need different tumbling techniques. The Victorian cichlids I have all produce small eggs. I have found that bouncing them gently on the bottom of the tumbler works best. Cyprichromis eggs are larger and shouldn't "bounce". These eggs do better if they are just gently moving on the bottom of the tumbler, sort of swaying back and forth. Malawian eggs are the most forgiving for tumbling experimentation. I have gotten good broods using the bouncing technique as well as the more gentle swaying technique.
I am currently using two types of tumblers. The first, my original, has a square box that the eggs fit in. The air supply above the box draws water through the plastic mesh on the bottom and easily bounces the eggs. I use this tumbler almost exclusively on Victorians. Air flow is regulated with an air line valve. This is very easy to use. My other tumbler is of a different design and requires more air than the first to work properly. This tumbler has slits on the bottom and on the sides of the egg chamber. It works great for that sway method, as it would require a lot of air to bounce the eggs. Presently I have a brood of Metriaclima estherae developing in this tumbler. They are nearing release and this is the first time since I began tumbling that I have had every egg from a brood hatch. I hope this means I'm getting better and that this is not just luck.
For the record, I've tumbled 11 batches of eggs so far: Haplochromis sp. "thick skin", Haplochromis sp. "rock kribs", Metriaclima estherae, Cyprichromis leptosoma, Iodotropheus sprengae, and Aulonocara mbenji, all successfully. The lividus haven't spawned again yet.
If you decide to try tumbling eggs, be prepared to experiment a little with the first couple of broods. Air current, temperature, and type of egg all have some bearing on the success you will have. Good luck and happy tumbling!