Hi Hammy I know u bought the Tunze Magnet... remember to always make sure so sand or stones sandwiched between the magnets, especially when u reach near the sand bed. Also do tie the string that comes with it... that way, u can always separate the magnets to check for hard particles.
For cleaning the external glass, I avoid using any form of chemicals. Just water and a nice Japanese glass cloth (bought from Taka). Works for me so far
Not too sure about Credit Card as old cards tend to peel off... although not very visible to the ###### eye. And have u guys bend a card till it folds before? The smell is Very strong chemical smell... like burning plastic. So... be careful... just my 2 cents anyway