rash decision when treating ich killed all my snails and crabs

I have a 55 gallon salt water tank (no corals but I'm wanting to get some in the future). I got a powder blue tang about a month ago and I noticed he had ich a few weeks ago. I freaked out a bit not wanting to lose a $60 fish and got some copper treatment to get rid of the ich. Unfortunately I did not research enough before foolishly putting the treatment into the main tank resulting in the death of my entire clean up crew. thankfully my powder blue is completely healthy now but I'm left with no clean up crew and was told by my local fish store that i cannot ever put crabs or snails in the tank again. i have plans to build a 250 gallon tank after I move into my new place but it wont be complete for at least 6 months from now and i don't want to leave my tank without a cleanup crew for that long.

Is there anything i can do to make the tank safe for snails and crabs until I am able to move them to the new tank? I have thought about setting up a smaller tank for the fish and tearing down the setup to clean it but would prefer to leave my tank set up.

Any advice is appreciated!