I use two on my 100gallon. The drawback is: not enough capacity. It will work well on a small tank. More useful for dosing micro/trace, because you can't dissolve much macros(NO3 and K+) in it.
I estimate the container volume is about 80ml, but the fillable/dosable volume is only 50ml or so. That's because you can't fill it all the way to the top and when the level falls too low, it dispenses less.