All the cichlids you list are hard water appreciating cichlids.
My first reaction, is you need to do larger water changes to maintain a higher and more stable pH, and alkalinity. Although the tap pH is 7.4, if your tap waters alkalinity is low, it won't buffer the fish urine and other metabolism by-products sufficiently and fast enough with only 20% WCs. More and larger water changes would remove those by-products better, giving lower alkalinity less to try and handle.
Adding a buffering type substrate like crushed coral (aragonite) or crushed oyster shell could help.
If you have a sump, you could drop a large mesh bag of crushed coral or shells where the flow is strong, or if you have other type filters put mesh bags of the crushed coral in them, that will also act as biomedia.
I used a fluidized bed with aragonite as bio-media to help buffer, the reactor was about 4 ft tall & 8" in diameter, and held a few pounds of aragonite, as it was dissolved by acids I'd add more.

Adding a half cup of baking soda to your replacement water could also help.
I was taking care of a 10,000 gal swimming pool for a while and found I could keep alkalinity and pH stable by adding a box or 2 of baking soda every other day (and especially after a rain event)