UV sterilizer

i am looking into possibly getting a uv sterilizer i notice that most of them are some sort of canister like contraption. I found one that goes directly into the tank similar to a tank heater is this bad, good, okay?

here is what i am looking at
$17.10 3w
http://dx.com/p/3w-uv-light-lamp-sterilizer-for-aquarium-black-ac-220-240v-2-flat-pin-plug-150cm-cable-122517?item=6

$20.70 4w
http://dx.com/p/4w-aquarium-submersible-uv-sterilizer-light-220-240v-121949?item=4

$22.70 6w
http://dx.com/p/6w-aquarium-submersible-uv-sterilizer-light-220-240v-116850?item=3

thanks
Dan

I have never seen UV's like this without a pump or case! I use the green killing machine $40 for up to 50 gal $56 for 100 gal. I have been really happy, and they have been going for about 18 mo without a problem!

I've been told anything under 29 watts is not strong enough to kill off disease, also flow rate and tank size matter. I have an inline pond UV filter that I run off my canister filter for quarantine. I don't use it after the initial quarantine. For breeding purposes, fish broods raised with a uv grow great. If I go to sell excess stock, being raised in such a sterile environment means that when they are introduced to a new environment they quickly succumb to disease because their immune systems have not been developed.