A blue glow can be seem from the end caps of a UV filter, is this dangerous?
No. It is designed in this way to assure the owner that the unit is on. The plastic cups filter out any harmful rays.

Where should the unit be placed?
In a dry position with good air circulation – beneath the tank if possible. Never position it under the tank hood!
When the tank is dark, a blue glow can be seen emanating from the tank return pipe from the UV filter; is this normal?
Yes perfectly normal. It will do no harm whatsoever to fish or invertebrates as the harmful rays will be filtered out long as the visible rays can be observed.
Can the length of tube be extended by switching the unit for only 12 hours each day?
This practice is not to be recommended as, although the tube will be extended, the aquarium will be left unprotected for unacceptable periods. The whole point is to provide continuous protection.
Can cloudy weather be controlled with a UV sterilizer?
Yes. If it is caused by free-swimming algae or bacteria a sterizlier is usually very effective.
Some models claim to be suitable for both marine aquaria and ponds, can this be right?
Possibly. Tests have shown that the optimum cavity space for marine applications must be no more than 6mm and this could be used on a pond as well. But if the cavity space is 12mm, or even greater, it is suitable for pond use only. If the model you are considering does not specify cavity space, look for one that does.
Is there a simple way to know when to change the ultraviolet lamp?
Yes. Write the date for renewal on the unit and in an aquarium log or diary.
Will a UV steriliser kill the hair and slime algae already in the tank?
No. It can only kill the spores that pass close to the tube in the unit.