Amquel Plus.... emergency, or the prime stuff.
But as first mentioned hope you got the live stuff out. You are in a catch-22, you have stuff dying because of the elevated ammonia, but more will continue to die. You posted more loss, get it ALL out.
You need to get some serious airation in that tank. All these additives make the O2 drop significantly. Cheap double bubbler at wallymart.
Big boss is right, but I did manage it for about 5 years in a 55G, 3 yrs on a canister. I only had 5 fish in the tank at any time... MAX. I had two pound of rock/gallon, and a DSB of extra fine reef sand. A HOB skimmer is a MUST at a minimum. Suggest the Remora 60G, and a HOB filter, ditch the canister=nitrate factory.