I remember it was fairly wide spread years ago... I'm wondering if anyone is still doing this or not. Also if you are which supplement are you using and what amounts?
I don't dose vitamin C but I do feed my tank a small amount (2 or 3 ml) of ascorbic acid (OK, it's vitamin C) every 2 or 3 days when I feed the fish, corals and anemones. It is a small amount even for a 90g system, and I do it more for the vitamins than it is as a carbon additive. But I don't think it's a big deal and I don't think anything in my tank would change without it.