Cool tube

I wana buy a cool tube, but i don't know what difference makes the 125mm comparing to 150mm. It's the diameter? How i discover which one i need? The 125mm or the 150mm...
Also, is there necessary a little vent at the bottom of cool tube? Or is just if you want to attach or not...

Thanks all

r u going to connect it to a fan?

i think about it and realize that i may not needed as i'll grow in a basement where the temp is around 22-24 Celsius, and the room has a plenty of space, so the cooltube it may not be necessary anymore. Or, what do you recommend? I took this decision as i have read in some forums that the cool tube is needed when the space is a problem, and the temp, which are not for me in the basement. so i decide to buy a kit with Osram NAV-T bulb, 400 HPS. I already finished 2 grows with a Philips Master SON-T Pia Plus 400W HPS, and i decided to use a Sylvania Grolux 400 HPS, but the shop told me the Osram NAV-T will be better. What do you think?

You are correct. A hooded fan is measurably more efficient at reflecting light than a cool tube. Sometimes there are no other choices in a tight space though. And glass takes away a few more percent so with your temps, just get an unenclosed hood.

As far as lights if you are happy with the Son-T stick with it.

what do you think about this ?

"Energy-saving lamp Envirolite 200W 6400K including 40x50cm reflector and side parts" (for Veg)

I'm a HID person, but that hood looks engineered for reflection unlike many cfl set-ups.

Yeah I prefer HID but I love CFL's too, That light looks good with the reflector on there.