Sorry to hear about your fish.
A heavy substrate vac would be smart. The pollution in the tank would be the probable cause of death.
As a side note, try using a 7, 14, 28 day pill holder and put in just the amount of food for each day next time you need to go away. Peace of mind
While you would be doing the vac, you would be doing a pwc
, and then maybe again until the scummy stuff is gone out of the substrate. You can do it all at once or a couple times so that all the muck is out.
Think of how you could be getting the water which is inside the tank changed out 100%. Which would be more than just a couple 50% changes, maybe 4 times the 50% amount.
Your aim is to get clean with out really scrubbing off the BB.
Test and make sure the water is perfect parameters. Then start feeding pinches of food every couple days like you are feeding the fish.
After a week do a good rinse out on the filter pad / media/ filter with tank or treated water to preserve your BB. You want to get out the poo and stuff but not scrub hard or damage the BB colony.
The idea is to leave the good amount of BB in the filter after the serious vac'ing to keep up with the feeding while you wait for the new fish.
Start looking for your new inhabitant.