Help me figure out why all my fish died

Sorry this is so long.

It started about a week ago. I lost 2 Perc. clowns & 1 royal gramma. 1 of the clowns was the very first one I got back in December. The other clown , had a few months. The royal gramma I've had a little over a month.

Last Wednesday I scooped some sand out of my tank to seed a new tank. The next day I woke to a dead clown fish (the 1 I had a few months. In a panic I dead a five gallon water change, I also added Mela Fix & Sress Zyme by Aquarium Pharmaceuticals. I usually try to avoid using chemicals. Some of the live rock did get moved around a little during the water change but nothing major. Everything seemed fine that Friday.

Then that Saturday I woke to a dead royal gramma, which seemed fined the day before & was eating well. Then noticed some white stringy stuff on my oldest & last clown. So I did another water change. Then had to take the family out to my sisters ( 2 hours away ) for an early mothers day thing there. When I got back home that night that clown was dead too.


Only the last clown had some white stringy stuff on it. What was this ?

All water parameters checked out fine before & after water changes. I did switch from Instant Ocean to Reef Crystals, could that have been the cause ? The only other thing I can think of is when I scooped some sand out. Could I have kicked up some toxic gas. I didn't even realize I dug 3 times in the same spot.
BUT ! Why weren't my inverts affected ? all the inverts are still doing fine.
The tank has been running fishless now since that Saturday night.
I wanted to post this sooner but had to go back to work Sunday & I haven't had time to get on the computer.
Could I have just done too much all at once & stressed the fish out ? Or did digging too deep in sand bring up something bad ?


Any help is always appreciated.

Tank Specs link below not updated yet.

Thanks a lot OrionGirl & mogurnda.

I had plan to let it run fishless for a while but would a week be long enough or should I go longer ?

There was no Ph difference with last test but plan to do another test Friday.

Not really in any hurry, just everyday my 4 year old tells me we have to go get more fish. He doesn't understand yet.
I'm really liking the black percullas.
Did a water test today.

Ph= 8.2 last test was 8.0.

Ammonia = .25 it was at zero last week.

Nitrite = 0

Nitrate = 20

Sg = 1.022

I'll give it another week with a water change in between & see what happens.

I had to move rocks around B4 I did the test & that kicked up a lot of debries clouding the tank a little. Would that have any effect on test's results ?
My hairy hitchicker crab was pulling my blue leg hermits under his rock & trying to eat them, after rescuing the 3rd one I decided to take the hitchicker out & put it into the 15 gal that I have cycling. It's been running for 2 weeks
I didn't want harm the booger but I felt I had no other choice.

One more question , would I be a bad fish keeper if I used one of my fish that died to help cycle a new tank ?