Can leaving cat food tins in the fridge harm my cats?

I buy my cats tinned wet cat food to add to their kibble, but I only give them about a fourth of the can per day... the rest I put in the fridge with a piece of saran wrap over the top of the tin. Someone told me that leaving food in the tins while refridgerating can cause botulism...Is it true that can cause botulism, and if so... would botulism harm my cats or does it only affect humans?