I cant see it anymore, I have no idea where it is :(

If your BTA doesnt rest where you like you can always point a powerhead at it which will often encourage it to move. Once it does this and your happy with the location feed it and it should sit pretty as long as you stay faithfull with feedings

Oh something bad happened to my tank this morning and I think it is something to do with BTA!
5 of my fishes died at a same time!

I think maybe the BTA died and toxic my tank , is that possible ?

I did a 50% water change and my scooter and bio color bleny back to normal now.

Still something is wrong all my britle stars tons are out they are all over my tank even on my corals!

What should I do?

Ouch, that's harsh. That sounds like it died and caused a crash.

The small brittles are probably feasting on all the decayed matter in the water. Have you checked all your parameters?

Brittle stars all went back and every thing seems normal , all the fished doing well ,I do a 30% water change tomorrow again. I could not find ine of fish and the BTA still missing I don't know where it is I wonder if they cause another crash agin!:(

Any advise ?

If you can run some carbon, if the BTA did die/ is dieing it will release toxins. The carbon along with the water changes will help clear the toxins or any other chemicals that may have accidentally gotten into the tank.