I only mentioned the habitat/behaviours issue previously, so I will expand on that for you. First, on the parameters, without knowing the GH I can't say who may be suffering here. GH is more significant for fish than pH, but having said that, the pH can't be way out of bounds either. GH involves the mineral salts in the water, particularly calcium, magnesium, potassium, and manganese, and this has a direct impact on a fish's physiological system. The internal processes that must function 24 hours are designed to function best within certain parameters. While some species can "manage" outside the ideal, some cannot. And while some manage for a time, eventually the strain this causes will take its toll. All of this is usually unseen by us, until it is too late; the fish is suddenly not acting normal, with no apparent reason, or it just dies, also with no apparent reason.
Rift lake cichlids must have higher mineral content in the water than any other fish we maintain in aquaria, with reference to the fish's physiology. If we want the fish to be at their best, we must provide this. If this were me, I would certainly provide an environment with a higher pH (8 or above) which is fairly simple to do, as this is tied to the GH and raising this will correspondingly raise the pH. But there are very few characins for whom this would be manageable. Enough for the present on water parameters.
These two groups of fish come from very different habitats in terms of their aquatic environment. Rift lake habitats are fairly open. The rocky shoreline is about all there is. South American habitats could not be more opposite, and this is the environment required by all characins. They avoid open water like the proverbial plague. They remain within plants in watercourses having aquatic vegetation, or cluster around branches, along the edges covered over by marginal vegetation and overhanging forest, etc. This is why such fish lose their colour in relatively bare tanks such as those in the store. Psychologically they are in danger, and they know it. The fish cannot override what nature has programmed into them. This alone will cause stress, weakening the fish, and it will not be in the best of health because it simply cannot be in such circumstances.
When it comes to behaviours, we must recognize the pheromones and allomones that all fish release into the environment. These chemical signals are read by other fish. Considerable stress can be caused just from this, even if no actual physical interaction occurs. Rift lake fish need rift lake fish as tankmates, and characins do not.
I hope this explains things better, but don't hesitate to ask further.
Byron.
