Peacock Cichlid dead no signs?

So looking into my 125G today I saw one of my peacock cichlids dead on the sand upside down. No idea on how he died? No signs of anything or fins nipped. Now I do know we have 2 somewhat aggressive peacock cichlids 1 is more aggressive than the other and he is a pretty big bully do you think he could have been the reason? Stress related? He doesn't chase as bad as he use to with less cichlids.

The only other thing I can think of was our water change 2 weekends ago, we noticed what looked like mold under these silicone Matt's covered by the sand, so we removed them. We originally layed those silicone Matt's on the glass so the rocks wouldn't scratch and it and would keep the rocks from moving. I was afraid there could have been something mixed up in the tank after especially seeing my dragon goby surfacing to the top of the water which I know was not normal (we moved him into our 60G for now so he is safe and stopped swimming to the top gasping). So I checked the water and the nitrate was showing high? So we did another water change to be safe. But we realized our api test kit was expired and did in fact throw off our results specifically nitrate reading. Tested with a new one and all the tanks are perfect in everything as of now.

So im just wanting to know what could have happened to the peacock. I just always want to know how it happened so I can avoid anymore if I can.

Any thoughts?

Sorry for the really long post but I appreciate everything if you read all of that.

 

African Cichlids require especially clean water. A weekly 50% WC should keep things healthy.
A dominant cichlid can badger tankmates to eventual death.
Likely too many possible factors to positively identify the cause of death.

 

African Cichlids require especially clean water. A weekly 50% WC should keep things healthy.
A dominant cichlid can badger tankmates to eventual death.
Likely too many possible factors to positively identify the cause of death.

Yes we do weekly water changes 40G out and 40G in. So 30+% a week. Do you think I should do more? We usually check our water parameters every week.

 

I change out 50% of the water each week in my 120 gal. My fish remain very colorful & healthy.
An occasional unexplained fish death is just part of the hobby. That's especially true with African Cichlids.
If the surviving tankmates continue to do well, the Peacock's death was probably just an abnormality.

 

I change out 50% of the water each week in my 120 gal. My fish remain very colorful & healthy.
An occasional unexplained fish death is just part of the hobby. That's especially true with African Cichlids.
If the surviving tankmates continue to do well, the Peacock's death was probably just an abnormality.

Gotcha that what I was thinking. But I am going to start doing 50%