I have a 55 gallon aquarium that has been set up for nearly 2 years. I HAD 3 neolamprolugus helianthus about 2.5 inches long, 10 blue cyprochromis 1 - 1.5 inches and 3 synodontus catfish about 3 inches long. All were healthy and in great shape.
Water out of the tap is hard and in excess of 7.6 ph. I have a marineland 360 cannister filter. Substrate is echocomplete african cichlid sand. The only decor is cave rock or texas holey rock that I have had for more than 15 years.
I did a water change and filter maintenance on Saturday, doing absolutely nothing different than I have ever done before. I also did a water change in my outdoor tropical pond and in another aquarium with green severum.
Yesterday afternoon, I turned on the light in the african tank, and all the fish except the catfish were dead. Fish in the other tank and pond are all ok. All water parameters check out fine except for the ph which is about 6.8. Not sure how that is possible with the substrate, cave rock, and naturally hard water with a relatively high ph. Also, could a sudden ph drop kill all the fish so suddenly?
Thanks for any input you might offer.