store links aren't allowed...

What I implied was that the bong already had a fixed downstem (usually of the metal variety) and they would have a carb on them. If not, you need to drill one. I prefer sliders but I have a handful of carb/fixed downstem bongs that I still use.

And no, it's not worth it to replace a metal downstem with a glass slider if the bong has a carb. It's expensive, and you risk breaking the acrylic (I assume, glass usually don't have metal downstems) when you inevitably have to bore out the hole a little to get the glass downstem in there.

What IS worth it is if you have a bong with a glass downstem, replace it with a diffuser downstem, they are teh win. Small bubbles = more surface area to interact with the water = less soot in your lungs.