A waterbed conditioner is necessary to stop the growth in your water mattress of the micro organisms that found their way in there with the water you filled it with.
Cozy, warm environment
Once your waterbed is up to the temperature that you are happy with it is also creating an environment that is very conducive to the growth of molds, algae and microbes.
The water in your bed is fairly static. There may be a little circulation formed by the heater pad but basically the water is stagnant and the life in it can migrate towards the mattress walls where it can do damage if given the time.
As the mold and algae develop you will notice bubbles forming in the waterbed and start to hear them as the water moves around when you are in bed.
Time to act
When you notice the bubbles it is time to condition the waterbed. This is as simple as reading the instructions on the bottle so that you know how much to use, opening the filling valve on your bed and adding into the water the prescribed amount of conditioner.
To keep your bed free of molds, algae and microbes you should be adding conditioner periodically – at least once every six months.