Vitamin c is often present in new salt water and is quickly used thereafter. There have been documented benefits of adding vitamin c supplements to fish food (IE Selcon) to have reasonable health benefits. I saw, years ago where some aquarists were experimenting with it with some success. I just wanted to experiment tO see if I could see any visible signs of health improvement or coloration.
Bang, you asked how will I know and to tell yah the truth, I don't. Lol. When I get up to the 5000mg mark per 25g of water, if I see improvements then I will continue increasing the dose. If I see degradation of water quality or a decrease in overall health, I will decrease the dose.
In humans, an intravenous drip of 50,000 to 100,000 mg of vitamin c a day has been documented to reverse and potentially cure cancer. So we will see.