Help tank crash, 1 fish left

[Kendalls_Reef]

i honestly have no idea what happened. everything was perfect until last night. i used an algae scraper to clean the glass and did my usual water change, exactly how i always do. i left for a few hours, then came back to feed my fish which was when everything happened.

i noticed my arrow crab was dead. and just as i went to go get the net to scoop him out, my african firefish started spazzing-- swimming in circles, trying to jump out, crashing into the glass, and then he died. my lawnmower blenny then did the same thing. before anything more could happen, i transferred my lawnmower blenny, green clown goby, and engineer goby to a Styrofoam ice chest where i ended up keeping them over night because i had no where else to put them. i managed to put together a quarantine tank this morning, and my engineer goby is now in it but i have a feeling he isnt going to make it. my clown goby and lawnmower both died less than thirty minutes ago.

i know the QT tank is small, but my engineer goby is only a baby, and he has a rock in there that he is currently hiding under. all my hermit crabs and snails are dead as well. i have no idea what happened. i have a feeling the engineer goby is going to die, as the water is super cloudy considering its not cycled as i quickly put it all together in an effort to get him into "clean" water rather than the tank, which i did a huge water change AGAIN. i'm afraid tp put the goby in the tank though, as i am trying to keep him alive. the fish have been in the tank for a month, and its been empty for four months. everything was perfect until last night.

i'm not sure what to do. i feel like im too inexperienced to have a SW tank and if my engineer goby dies, i will probably turn it into a freshwater tank as i know how to care for those without any problems. any advice on how i should keep my engineer goby alive is greatly appreciated. thank you.

 

[Autumnsky]

Where did the water come from? Was it pre made for you from the store or did you make it and let it sit for any period of time?

Did you test the salinity? Use tap water and forget to dechlorinate it?

 

[54seaweed]

what size tank is it ?
how old is it ?
was it properly cycled ?

please post all your water parameters along with everything you've done up to the crash
and what the stock was , along with a rough Idea of what you have done to the tank in the last 72 hrs including any livestock you may have added

NOTE:
(This is only observation and thought of what you told us this may or may not be the cause)

hermits and snails are tough little inverts if they were all dead I would lean more towards something killing them , than from that they fouled the water causing you to crash leading to killing your fish .