GE I 100% silicone for windows and doors is fine for aquarium use. Generally if a silicone says 100% silicone, or has an FDA food safe approval, it's ok to use and does not contain fungicides. If the product states that it is mold and mildew RESISTANT, it's still ok. ALL SILICONE is mold and mildew RESISTANT, because it's silicone. However, if it states mold and mildew PREVENTION or PROTECTION, or gives any kind of guaranteed mold and mildew free for X years, then it has a fungicide in it. Most window and door silicones will not contain fungicides, most kitchen and bath products will, because guess what, that's usually where you have a moisture/mold problem.
If you are still in doubt about which silicone to choose, compare the ingredients on the back with an "aquarium silicone", generally speaking there's no difference.
The other issue that most people over look is what kind of byproduct is produced during the cure. You want an axetocy type silicone, anything else is not good for an aquarium. Acetoxy cure chemistry produces an acetic acid byproduct, which smells like vinegar and is corrosive to some metals such as brass and copper.
Chemicals to avoid, ethylene glycol (toxic), acrylamide (neurotoxin), gamma-aminopropyltriethoxysilane (also known as (3-Aminopropyl)triethoxysilane) (toxic to breath, touch, or injest), formaldehyde (we all know it preserves, but it is used in adhesives/sealants as a disinfectant/biocide, it will kill bacteria as well as fungi and their spores).
Grab your silicone of choice and look at the WARNING! if it list a bunch of chemicals and then something like harmful if swallowed, or absorbed through skin, it is toxic to your fish!
My tube of GE Silcone I Window/Door/Attic/Basement in clear says; "WARNING! Uncured product causes irritation. May cause headache, dizziness, nausea." This is due to the acetic acid released during the cure. There is no mention of kidney, liver, or cardiovascular failure. Basically don't huff the stuff and don't get it in your eyes.
I have another tube of DAP windows and doors in clear and it lists the chemicals i mentioned above as well as many more. it also states adverse reactions it can cause that make me want to not even sit next to it, let alone use it.
BTW, you can find GE I at home depot, lowes, menards, walmart, meijer, and probably target.
My knowledge comes from years in retail, experience in construction/contracting (whole family in that line of work), lots of research, and a chemist for a sister.