Strip it
Strip ALL Silicone. Old silicone, or even newly dried silicone cant adhere to new silicone. It has to be a "All or nothing" job. I used an exacto blade and some 100% acetone (evaporates damn quick!) to clean out the old silicone. depending on how old the tank is, you may want to gently poke the silicone between panes to see if that is still good. (if it crumbles or balls out really easy, time to replace>) Good luck!
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