Fresh water is used prior to starting a new smoke session. This ensures that you get the most flavor and bang for your buck. Old water in a bong can ruin a good hit and even make you cough more. It will essentially taint the overall flavor of your hit and impact the enjoyment of your session.
Using clean, fresh spring or distilled water is most ideal for a bong. Using other liquids not meant to cool down or filter smoke should not be used. Fresh water will allow the smoke to pass through without trapping the terpenes and oils within the smoke. Dirty water will lessen the amount of good stuff in your hit.
Ideally, you should strive for changing out your bong water after every session, prior to a new bowl. Simply empty out and rinse your bong after using it. This will keep your chamber clean and your sessions enjoyable. If you leave your bong water in the chamber after a session, it will turn stale and start to stink.
While the chamber is still warm after a session, you will find that rinsing it cleans off more residue. If you wait too long after it has cooled, it will be harder to clean. This is especially true with glass bongs that are hard to clean. Do your best to rinse out and maintain your bong after every session.
Small bongs will need to have the water changed more often. This is because a smaller chamber will get dirtier faster. Big bongs can hold more water and filter out more smoke. The longer your sessions, the more water you will need before having to clear it out.
If you notice your water is very dark or cloudy, simply empty it out and fill it with fresh water. This will keep your hits clean and flavorful. Clean water will also filter out your smoke much better. Small bongs are more convenient but require much more maintenance being easier to get dirty. Simply rinse out your bongs after every session.
You should strive for changing out the water in your bong as often as needed. After your smoking sessions, this should be mandatory, despite the fact that your water may appear clean. Over time, residue will accumulate along the walls of the chamber. Even if you took small hits, you should keep this in check.
If you plan to take multiple hits throughout your day, simply empty out the chamber and refill it when you are ready. This will make your bong easier to clean and keep your sessions running smooth. If you insist on leaving the water in your bong, that is fine too. Just know that you will do more cleaning in the long run.
Bong water can be very beneficial in a bong provided it is clean. Cold water is great for cooling down a hit while hot water can reintroduce moisture in the smoke. This will make the hit much smoother and lessen the chance of coughing. In either case, you will want to change out the water as often as needed.
Always keep in mind that you should never put ice cold water in a hot bong. You should also never put hot water in an ice-cold bong. Doing so can result in pressure that will ultimately crack or shatter your glass bong. Keep everything at room temperature and change out your water prior to every session.