The Mysteriously Dead Fish

[BigClownFace]

Hello, all. Literally three minutes ago my blue-green chromis died. I've had him for about six months and he's always been perfectly healthy, eating well, and swimming without any problems. He was living in a 40 gallon tank with a dwarf fuzzy lionfish, marine betta, snowflake eel, and yesterday I added a Singapore Angelfish. Before adding the Angel I did a 10% water change. When I got home late last night, I checked on the fish and the chromis had turned white and was lying upright on the ground. After having the light on for abit, he turned back to his normal color, but still remained on the sandbed. Two hours later, while the light was off, he turned white again. I figured it was just some sort of sleeping pattern, so I thought nothing of it. This morning I turned the light on and he was still white. Once again, he turned blue-green after a few minutes, but he seemed weak. I watched him for a while and he started to lay sideways on the ground, then get up and swim around, then lay sideways again. I left the house for about an hour and when I came back he was laying sideways under a ledge of live rock and breathing rapidly. I started looking up fish diseases online. He started swimming around again, but every now and then would flip, like he had lost control of his motor skills. Eventually he went into spasms, like he was having a seizure. Afterwards he laid sideways on the ground and died.

The nitrate and nitrite levels are 0, and the pH is 8.4. SG is 1.025, I plan on changing the water again today to lower it.

I'm not sure what happened to the little guy, maybe Velvet?
If not, what could've caused him to die?
Is the tank overcrowded? Is the SG too high?
Could he have gotten stung by the dwarf lion?
Could it have been caused by doing a water change and adding a new fish in the same day?

Thanks in advance for your help :]

 

[SkiFletch]

Tough to say but since you mentioned it I'll ask a question: when's the last time you verified your salinity measuring device against a known standard?