Better to put alot of bong water in or a little

Depends if I'm using ice or not. If not, I'll fill it up a little higher to cool the smoke a bit more. If I am using ice, I'll fill it to just above the highest notch in my diffusor. More water = cooler hits. However, water, in addition to cooling the smoke, removes some of the THC in the process. You normally won't notice a difference, because you're taking bigger hits, and everyone knows you get the highest when you take the biggest hit you can and still hold it in for 7 seconds. More than 7 seconds, you need to take bigger hits. Less than 7 seconds, smaller hits.

Personally I like to fill her up with a little extra water. This makes the bubbles bigger and they sound better. I also think that as long as the smoke is being filtered through the water, adding more water won't filter out more carcinogens since the chemicals are either water soluble or they aren't.