Mag can decline slower in some cases ( not all ) for sure. But if mag is not kept within the right range you would know that alk & cal will not balance or remain stable & steady. Once again when talking price of mag test kit why not make it part of your weekly routine. My tanks mag declines enough for me to have to dose everyday so yes I do test every week That way you can catch anything that is going wrong very quickly.
If you keep your Mag levels close to NSW levels, then yes, you'll probably have to dose more frequently... requiring more frequent testing. As Mag doesn't have to stay within a specific range (although often recommended) like Calcium and Alkalinity, I run my level a bit higher than NSW level (1280ppm). Mine is usually in the 1300-1400ppm range, and consumption isn't very high, which allows me to go longer between doses. Extra Mag doesn't hurt anything, and I've had mine as high as 2000+ppm for a couple of weeks while battling a briopsis outbreak. As recently as a month ago, I accidently overdosed Mag by 3X when using a new measuring device that I misread... with no adverse side-effects.
I'm slightly aware of the role of Mag (see post #7), but I'm also aware that the upper range isn't quite as critical as Cal and Alk. As long as it doesn't fall below 1280ppm, you'll be okay, and as long as it's not extraordinarily high, it won't throw Cal and Alk too far out of kilter. It all boils down to what each individual tank consumes/requires. The only way to know this is by testing. After awhile (sometimes years), you'll detect a pattern and know how often to test, and what to test for.
