UV light question

[yepreef]

So i am new to UV light, my question is that can i leave uv light turn on while doing water change or feeding with return pump off? I have UV installed vertical as picture. Thank you

 

[Orko]

that would prob be a bad idea as the quartz sleeve and lamp will heat up then when you water gets turned back on .. it may crack. UV's norm use the water being pumped through them to cool them...