What Size Magnet Can ERASE a Credit Card?

Once upon a time I was  computer repair tech. There was a product named Magna-See that we'd use to inspect track alignment/signal-quality on the read/write heads in magnetic tape units.  One would dip a piece of recorded mag tape in the fluid, remove it, and let the solvent evaporate.  You could see the Flux changes on the tape thanks to the incredibly small magnetic particles suspended in the fluid & that adhered to the mag tape. I don't think that product is around any more but there is one named Kyread SDF-130A that looks like an equivalent.  That might be a handy resource for any future experiment where you need to visualize magnetically recorded signals.