Buffer is right about the silicone sealant. If it has a mildewcide you should not use it. If it says it is paintable you can not use it. The catalyst in silicone caulking is typically acetic acid (IE vinegar is a natural form of acetic acid). Just as there are hot mixes in epoxies and fiberglass resins there are also hot mixes with silicone. Hydrochloric acid and worse are used with these silicones. The working time is shorter before skimming occurs and the life expectancy is shorter with hot mixes. Dap is a good and cheap aquarium safe silicone. Most GE and other high priced hardware store brands are hot mixed silicones, but they also make aquarium safe silicones. Mildewcides are almost always proudly listed as mildewcides on tubes. If you are worried about residual acids wash your silicone repairs down with baking soda water before filling tanks. I like old threads as long as they are short threads.