Posted: Tue Jun 30, 2015 8:39 pm Re: Is Prime safe ?
In my experience, smaller ones weekly help keep the water chemistry more stable. Nitrogen-fixing bacteria have 2 big threats: Chlorine and starvation. New, clean water poses both threats--even if it's good for the turtles. Chlorine can be avoided by a carbon filter or dechlorinator if necessary, and in long-established tanks, the bacteria actually develop a film that keeps them pretty safe from chlorine's effects. However, what we can't avoid is depriving the bacteria of their food source as a result of changing the water. This usually leads to a bacterial die-off, as well. This is why I'm not a huge fan of big water changes, which I've seen many people do, only to unintentionally start mini-cycles (and end up with unexpected ammonia/nitrite readings). At least, that's my theory...
Another option, which I'm sure you've already considered, is to add more water to your tank. I know you have 4 decently sized turtles, as well as some fish--you might just be overdue for a volume increase to keep up with the extra ammonia they all produce.