As a note: 2 quarts is approximately 36 spheres
Recommended minimum usage is approximately 1 quart per 100 gallons of tank water.
This means 18 spheres for 100 gallons, 9 for 50 gallons, so on an so forth. Does this means this is ALL I'll need for bio filtration? So if I have a 18 gallon tank, adding 3-4 spheres is all I'll need for biochemical? Wow!
Bingo. Just make sure you've got them in an area where they receive decent flow, like in your filter. Unless you're using them to reduce nitrates. Then you don't want them getting much flow at all
Another neat thing you can do with them, especially if you're like me and are always starting new tanks, is use them to seed a new tank VERY quickly. I keep a couple of them in a little mesh bag in one of my well established tanks, and just drop one into a new tank when I set it up. It reduces the cycling time dramatically.
For example: I had an old, neglected 30 gallon saltwater tank that I was getting ready to take apart and upgrade to a 72 gallon. I used dry rock to construct the layout, and added 5 MarinePure Spheres that had been sitting in the 30 gallon for a couple of weeks. My cycle time on that tank was just about one week. I'm still going through the algae outbreak fun, but my ammonia/nitrate/nitrite cycle only took a few days.
Seriously, there really isn't a comparison to MarinePure out there. Sure, it's not a flashy, fancy upgrade to your tank like a new light or new filter pipes, but dollar for dollar I don't believe there's anything better you can do for water quality.