Your tank is big enough for the cardinal. He might have been sick or he is very stressed. They can hide in very small area so it may still emerge later. I once killed a sixline wrasse. by carrying a rock in my sink, because I wanted to scrubb the bubble algae. I scrubbed the rock, put the rock back in the tank, and when I went back to sink and found my sixline dead in the sink, probably killed by the cold tap water I was using to clean the rock. I never saw it, and the rock was not very big.
Hermit crab, nassarius snails or shrimp would be better to clean the dead fish. Your snails are good for algae.
Hermit crab, nassarius snails or shrimp would be better to clean the dead fish. Your snails are good for algae.