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You can not use any of the silicone with stuff that is 'mold free'. That stuff will leach into the water column. If I remember correctly, it'll basically uncycle your tank.
I personally sealed 2 aquariums with GE 100% Silicone and one of the aquariums has been up and running going on 4 yrs with no issues. Just make sure it has no anti mold inhibitors in it.
Best bet is go to your local fish store or even amazon get some silicon sealant that is approved for aquariums no point in taking chances with your fish.may cost a little more but you know your fish are going to be safe
I've resealed many old aquariums with Ge silicone 1. Works great. Use a rubber glove on your finger, get it a little wet in clean water, and create a smooth bead line after applying