I've been using GHS Fast Fret now for a few months. I give the strings a rub with it after every time I play my basses.
I can't say I can notice any negative difference in playability or sound.
The only thing I can feel is that my fingers slide across the strings a bit easier after using it.
Try some Bass Brite wipes. www.bassstringsonline.com
I've been using them on occasion more recently. They certainly seem to clean the strings well. There's always grey residue on the wipe after use.
What works for me is:
I use GHS Fast Fret every time I play.
One in about six plays I'll hit the strings with a Bassbrites wipe to clean them thoroughly, followed by Fast Fret again to protect them.