I use diluted chlorhexidine & a soft brush.
You can use diluted vinegar, or dilute some baking soda or salt in water.. or just use the brush.
Whatever the case when they feel slimy you know that's a bacterial build up & so it's all about getting that off of them.
I use a dog toothbrush, it's softer than most other brushes I've found.
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PHRatz