Silicone I gives off acetic acid while curing. This is the vinegar smell. A friend of mine worked in Axelrod's aquarium building company many years ago and still has problems from the damage it did to his sinuses and lungs. Your body gives you no warning of the dangers of breathing in acid vapor. The hazard of smelling this stuff prompted the development of Silicone II which instead gives off ammonia. Your body knows about ammonia. II has about 80% of the strength of the original formulation, plenty for building a tank. Once the II is completely cured, it no longer gives off ammonia, but until then it is not safe with fish. If you do all your assembly and curing in a separate place from your fish, there should be no problem. However the company is off the hook with their disclaimer. I always use the original stuff but make sure have ventilation in the area, and a fan blowing fresh air right at my work site.