Water change error - I think anyway.

[newby01]

No, the under gravel filter was starting to come out of the sand, so I had to rebury it. But in order to do so, the water, sand and rock had to come out so I could rebury it.

Yeah, lesson is to ensure I check what the water is before I remove stuff, and then check again prior to putting fish back in.

 

[Suohhen]

So sorry for your losses and Iam amazed by your upbeat, open and ready to proceed attitude. Losing fish is incredibly hurtful and is typically the reason why many people don't make it past 1 year in this hobby.
If there is a lesson to be learned it should include the need to go back to the basics. Forums are great for discussion but proven best practices are the way to learn. One place to start is the sticky posts on this thread, there are also a ton of amazing people on YouTube, if your into podcasts Reef News Network is an amazing source for the basics, the book The Reef Aquarium is basically the Bible on this stuff.