From the photos, I would not suspect calcium deficiency here. Something could be eating the leaves--I see this in my 90g with Botia kuborai who munch on sword plant leaves, though very little and not enough to bother me. None of the fish mentioned seem likely candidates. Snails maybe, but depends upon the species. You mention "small" snails, like pond or bladder maybe? These do not eat healthy plants but will munch on dying leaves. My tanks are full, literally, of Malaysian Livebearing and pond/bladder snails and no plants ever get eaten. Mystery snails I am not familiar with... ?
The other option is a nutrient deficiency. It is not much, since the damage is so little, at least in the photos. I believe you mentioned that no fertilizers are added. This can work, depending upon fish feedings and water changes. But the trace minerals are not always present and you do have some fast growing plants which thus take more light and nutrients. You might want to consider a comprehensive supplement like Seachem's Flourish Comprehensive Supplement or Brightwell Aqwuatics FlorinMulti. In a 29g, you would only need 1/2 teaspoon of the Flourish Comp once a week. I would try this and see if after 2-3 weeks the problem is solved. Flourish Trace might be sufficient, but it lacks iron and that may be part of this.
Byron.
My snail population is quite literally taking over my tank though xD I tried cutting back on feeding and the population is still steadily climbing. I try to remove clutches when I can find them. But I don't want to completely kill them off because they really do help out. 
