I live in Pensacola, FL and have had my 23.5 at a slip for 3 years as of this summer. I've pulled it twice in that time (once for a storm that ended up being nothing and last summer to repaint the bottom). I have never added anything to my ballast tank. I let it fill with the salt water in Pensacola Bay and close it off. The Marine science I know (which granted isn't a lot, but Google is often my friend) seems to indicate that without light (or hot water vents at significant depths) very little will grow in a marine environment. I have never had any issues with a smell in the cabin and when I pulled her last summer the water drained quickly which seemed to me to indicate there was not significant growth inside that was restricting the opening to the tank.
I hope this helps. I've seen the same posts you have about adding bleach or using chlorine tabs. Personally, I have not seen the need, and it seems to me that the chlorine would damage the seal to your tank and harm the environment when you drop your ballast, IMHO.
