the best way is to replace the in tank fogger with an external ultrasonic humidfier(fog is made the same way) pipe in the fog with some pvc pipe, this will give you
1. large reserve of water
2. better control of fog amounts(unit has controls and or add timer)
3. no pool of water in the tank that heats up and has to be cleaned out
4. option of fogging several tanks from one source
try to use some form of RO or distilled water if you can