Cichlid skeletons can change (evolve) in as little as a year or two, depending on natural or changing conditions, as is spelled out in the study below, where putting a dam in a S American river, reduced flow drastically and altered the inhabitants.
Evolution in cichlids video
Most riverine cichlids live in much stronger flow conditions, that are almost impossible to attain in aquariums, so in a situation where aerators, or simple dribble from the spray bar of a canister flow is all that is offered, their bodies adapt to the much less strenuous life style, and they and their fry become weak compared to their wild cousins for generations to come.
I have snorkeled in rivers where I've had to hang on to mangrove type roots to not be swept downstream, but where the local cichlids are barely phased at all.
In nature only the fittest survive, which might only be 1or 2 individuals in a spawn of 1,000.
We, as aquarists often allow 50 or 100 of a spawn to survive, allowing many sub par individuals to procreate down the road.
Or.....When we breed cichlids only for color, or humps, or other such outward traits, this can de-select for less visible but important traits like health, immunity, temperature change tolerance or robust or aerodynamic body shapes that cope with moving water.
The EBJD is a perfect example, selected only for its predominant blue, its general health and even ability to defend itself like a normal JD have been compromised, its ability to fight off disease is generally poor.
