I only recently became aware of doing smaller water changes vs the weekly 50% I’ve done for the 3 years I’ve been in the hobby. This new information comes from a few Angelfish breeders I am in contact with. I have purchased their fish, and I’m listening to what they have to say regarding water changes. Other methods of single weekly large water change came from African Cichlid type people in my early fish forum days. Different fish have different tolerances so what might work for some may not work for others. It sounds like you suffered from TMI ( Too Much Information) from too many sources. Sadly, with the lack of good Mom & Pop shops these days, there are many people/ experts online who give advice based on limited experiences so it becomes harder to know who to listen to. I've always been a big supporter of "The best information is local information." Sadly, that's not always available.
I was advised today to do a 75% change today and the tank where all the fish died , not from one of these Angelfish breeders but from a well respected person in a large forum. Of course that change was already in progress when his message came in. For the three years that I have focused on these African reflex cichlids, I’ve gone with the big changes and have not tested the water prior to a change. in fact, the readings were so stable over time that I came to do them less and less frequently. The tanks were established, and there was no reason for these parameters to suddenly jump. This is a great example of why you shouldn't change your routine just because the tank and fish are older. An established tank does not mean it's a healthy tank. There is even something called " Old tank syndrome" that happens when the tank water gets old and with a reduction in water changes, nitrates get high as does phosphate level, KH gets lower and Ph drops. You can't see these things with the naked eye, you only see them with tests. Then you do a massive water change and the Ph jumps up which causes shock in the fish. ( I'm not saying this was the only cause but could be a contributing factor. )
Obviously, something happened last night. All fish looked good when they had their end of the day meal. The survivors continue to look good and are active and eating. I believe only a necropsy could determine the precise cause of death. It had to be something and I am grasping and struggling. I agree, we are only going to be guessing at possible causes while only an autopsy can definitively tell you what happened ( unless you have a video of the tank that was running overnight so you can see what happened and when it happened. )
Tell me if you agree with this, or maybe you already told me, I am so rattled and confused now that I don’t even want to go back and reread everything. I’m being told by one of my advisers that is not likely a contagious condition. I was terribly worried that my nice water exchange system had become contaminated, but this highly experienced person does not think so. He thinks the most probable cause was low oxygen in the tank. I had two small stones one at each end. Today, after the event, I replaced both with much larger air stones. They are quite massive and heavy. I’m going to order more. Yes, I do not think this was disease that killed the fish. If the cause was low oxygen, the fish would have died with their gills flared open. If they didn't, it wasn't because of low oxygen. You need to be careful with air stones. You can over saturate the water with oxygen. They do make " dissolved oxygen" test kits if you are concerned that you do not have enough aeration. Too much oxygen can cause bubbles to form in a fish's skin and around their eyes, which can lead to gas bubble disease. This condition is rarely curable so should be prevented more than thought of as curable.
Yes, the food could have somehow became contaminated. As soon as you mentioned that prospect, I threw the big bucket of it away. It was almost empty, and I have another. If other fish were eating the same foods, it may not have been the whole container. It just seemed to be the one common denominator to all the fish if it wasn't the water.
