I try to stick to bi-weekly water changes of between 15 to 25% depending on what the tank looks like. I usually test my CA, MG and ALK every other week as well. Unless something doesn't seem right, then I test more often to get a better handle on my dosing regiment. For example, lately, with my new tank I've had to test more often to gauge my ALK consumption. It seams that just because I doubled my water volume, it doesn't mean I simply double my dosing. If only it were that simple.
