Hello, please tell me what you use and your opinion on it ... I am thinking of getting one for my 55 gal tank but not sure which to get. Thanks in advance

I dont use UV's on my tanks inside, only on the ponds outside, since the Florida sun makes green pea soup out of them otherwise.
And I have to say, I am very happy with the Aqua UV sterilizers (I think thats how they are called). They come in two versions - with and without wipers which mechanically wipe off the bulb inside without the need of opening the whole unit.
I got the one without wiper since the Aqua is very easy and fast to open and clean anyway. I got mine from Pondkeeping.com, which advertises on Koivet.com, but you probably could get them at Petsolutions or the pond sections at DrsFostersmith.com. Eric Johnson did a great job on recommending the right size for my ponds (POndkeeping.com)....

I checked at Drsfostersmith.com and see they have many brands but what I dont get is how the measurement works, some is 9w , or 18w or 30w etc .... What do you think will work on a 55 gal tank ? Thanks

NEver had one on a tank, but Hidr has, and she is very happy with the results. If you click on her name somewhere here, you can send her a private message if she does not see this thread and ask her.
Strangely enough, they recommend stronger UVs for tanks than for ponds, I dont know why. I have seen UV's advertised that you can use on a 75 gl tank or a 300 gl pond.....
Maybe the customer service at DrsFosterSmith.com can help you out, they are very knowledgeable...