Tad, thank you for the 'field report'!

You've brought up something that has been mentioned here and there but not as often as the 'sand never needs to be replaced' 'riff'.

I think that you were very astute to see the co-relation between the filthy sand you were left with and your problem - most probably (from years of neglect, or improper maintenance) you have saved yourself #s of headaches by doing this -- CONGRATS!