That is my tank by the way not tap
We also need to know the tap water values for all three. I have no idea of your level of understanding here, so I will briefly explain why.
First as to the possible difference between source (tap) and tank. The initial GH and KH governs the pH and the degree to which the GH and pH may fluctuate, if at all. This can be due to natural water chemistry and biology, or "targeted" by the aquarist by adding substances that target one of more of these values. The higher the GH and KH, the more stable the pH and it will resist any attempt to adjust it. There are ways to do this, but that gets involved and I think we can go on for now anyway. But this is why we need to know the source water parameters; something in the tank may be affecting things, but the source numbers will pin this down somewhat.
Which brings me to the numbers for the tank water. The pH at 7.6 if this is stable week to week is adequate but the absolute lowest it should be. The GH at 85, which I assume is ppm or mg/l (these are identical values) rather than degrees which frankly would be impossible, is way too soft for rift lake species. This can easily be rectified by using a calcareous substrate such as aragonite sand. This is the least expensive, as the substrate will literally last for ever as far as we are concerned. A second method is to purchase "rift lake salts" and add the appropriate amount to the water at every water change. This is obviously much more expensive, and it introduces the issue of having to prepare water outside the aquarium. The "salts" are not common table salt, sodium chloride, but the salts of minerals like calcium and magnesium, so do not be mislead into thinking aquarium salt or marine salt will work, it will not and it will seriously harm the fish. Marine salt has sodium chloride as well as the other mineral salts, and aquarium salt is pure sodium chloride.
Now to briefly explain why this is critical to fish. Each freshwater species has evolved to function in very specific water parameters and environment. Their physiology is designed accordingly, and the metabolism will only operate properly in such an environment. [Having said that, some species have a wider tolerance for adjusting than others, and there are reasons for this built into the species itself, but this is not as much or as widespread as some like to think.] The more the conditions we provide differ from those in the fish's habitat, the more likely the fish will be stressed and more susceptible to disease. ["Conditions" here means the environment, i.e., the substrate, décor, lighting, water flow, other fish species, numbers within the species, and water parameters.]
The relationship of a fish to its aquatic environment is many times more crucial to its well being than any terrestrial animal with the exception of amphibians. Terrestrial animals breathe air, and aside from cataclysmic events, air is air almost everywhere, which is most assuredly not the case with water. This extreme close interconnection between fish and water is why the parameters are so important. As far as air is concerned, any animal could be moved anywhere and would still be able to function (subject of course to some other factors), but this is not the case for fish unless the water and environment is identical.
I will give just one illustration...pH. A fish must regulate the pH of its blood to match that of the water it lives in; this is partly because that water is continually entering the fish via osmosis through every cell and via the gills, entering the bloodstream. But to do this there are complex physiological processes that the fish uses, but the effective functioning of all this is also dependent on many other factors, many of which are also affected by the parameters.
Where this all leads is to your GH primarily. For the species
Labidochromis caeruleus It must be higher, at minimum 10 dGH (= 180 ppm). The preferred range for this species is 10 to 25 dGH, with a preferred pH of 7.7 to 8.6. The fish is not going to function normally, which means weakening and degeneration the longer it is forced into this situation. The other rift lake species are similar.
Hope this helps explain things. You mentioned fish losses initially, four with one being due to you...but these in themselves suggest problems and here the water parameters clearly were a significant part of that. Fish under stress as these are, exhibit it in one of two ways, increased aggression or the opposite, withdrawal. These are also naturally aggressive species, so that would only be amplified.