Healthy firefish died over night :(

[Grebex11]

Hey guys,

I've had a firefish for exactly two months. It's been eating ever day. I do a mixture of mysis, pellets, and flake. It even ate yesterday and then when I look at the tank this morning, dead. From everything I could tell he was perfectly healthy before turning up dead.

Only one other fish in the tank, a YWG I've had for 7 months. He appears to be fine.

This is a 15g mixed reef. Only changes I've made to the tank in the last week: added a green toadstool coral and a devils hand leather on Monday (4 days ago). I know these can release toxins but I've never had any issues with them before. Seems unlikely. I also started using some GFO on Sunday. Again, not something that could cause this.

Any ideas of something I could have done?

 

[Grebex11]

One other thing I don't typically do but have done once before. I was in a bind an used distilled water for top off instead of RO from my LFS. Again, I've done this before and have gone through the forums of people saying it's fine. But that is also something different I've done in the last week.