Despite Jim's success with using a float switch/powerhead arrangement, I would still recommend the dosing pump. There are many horror stories about a stuck float switch and a flooded sump. If you're just using water for top off, it'll knock down your salinity but that's it. If you accidentally dump a few gallons of KW into your sump at once, you could wipe out the tank. That's what happened to Minh in Corpus. I guess it depends on your tank size; if you have a 200-300 gallon system, a few gallons might not be a big deal. My tank is so small that even a 1 gallon overdose would probably kill alot of animals. The dosing pump makes it impossible to accidentally pump excess KW into your tank, and since it's a steady drip rather than a periodic stream, there is essentially no variance in salinity or ph.

I like these dosing pumps so much I'm thinking about getting another one to dose my home made 2-part Ca/Alk supplement. I think I need about 50 ml/day (about 2 oz) and the newer innovative aquatics pump can do that, it actually goes down to .5 ml/day per channel for 2 channel dosing.