Geoffkait, you are welcome.
Now here is a simple explanation from Isoclean itself of why replacement every now and then is necessary. I quote this from an audio article written by an oriental reviewer living in Seattle. I can actually read their language and translate them for you. There are of course sematic differences which cannot be 100% translated so bear with me.
"Because of the few samples on hand, I haven't directly compare AHP against Isoclean, but both are effective. According to the properitor Mr. So of Isoclean, their fuse have limited life, and even if not bow up, overtime the sonics with degrade, the primary reason being that the thin conductor core within cannot withstand overtime the micro heating from prolonged passing of current; or the instaneous heating up upon switching on of the equipment will cause change in the thin conductor core, resulting in the fuse losing its initial glamour and softening in sonics. Based on my observation over the months, the effect described is in fact true. Looks like we have to replace our fuse from time to time to keep up with the sonics".