First, your tank is beautiful.
Fish are like any other animal. They can get sick and die, even if we do everything right. They may not show any of the normal symptoms, and still get a viral infection, Bacterial infection, parasites, old age, or even cancer. It just happens some time. It's part of fish keeping and can be extra hard on new fish keepers.
It looks like you have checked all the important stuff. Maybe check your temp and see if it is where it should be for all of you fish. I will often double check my tanks with a different thermometer.
If everyone else looks fine, I would just keep enjoying your tank and keep a look out for typical symptoms before getting into making any changes or medicating. The best thing to do is to keep up with your routine feeding and water changes. Keep it simple and don't add extra variables.