But aren't the majority of salts present in rift lakes more like sodium bicarbonate (baking soda) as opposed to sodium chloride (sea salt), which is really just a trace element in the lakes?
Lake Salt is just an additive and keeps the ph where someone wants it. most people add aquarium salt for gp. so you basically answered your own question that the fish are kept in freshwater.
I have 2 prs in a 29, now down to 1pr. no lake salt or aquarium salt, just prime when wc ea week. they breed almost every other week. makes great feeders for my small dats. luv watching them hunt.
they are most likely eating food or dirt particles in the substrate.
with my convict pair the fry would follow the female round and she would dig up the dirt and the fry would eat most of it like a gravel vac. the convict fry also could clean the glass of any algae pretty well!