i am wanting to get a uv sterilizer, is there any good types out there? brands?

You need to give a bit more information. What are you planning on using it for, tank or pond, fresh or salt, what volume of water, what are you trying to achieve (fix), internal or inline, what budget do you have.

sorry for my discus tank which is about 250l i want an internal one.

and what do you want it to do?

to make sure Parasites, viruses don't harm my fish

Any thing over 18W I think will be fine, but really - you don't NEED a UV sterilizer.

I have one of those big 24W ones and all it does it sit outside the tank. Only get a UV if you have a specific need for it. Probably good to have one on hand but don't run it 24/7; there is no need.

Have a good WC regime and filtration system instead.

Fish are not designed to live in a sterile environment, I have heard suggestions that running a UV in a tank full term makes fish more susceptible to issues when you turn it because they are used to living in a sterile environment and loose their natural immunity. Unsure how true it is but it seems to have some logic behind it.

Do you have issues with parasites/viruses atm? If anything I would purchase one and just have it on hand in case you get white spot or something like that.

I would doubt that a UV "sterilizer" would actually sterilize anything. They would reduce the "live" load in the water column but the requirements for them to operate properly are pretty tight and I doubt those conditions are met usually.

Some UV information

http://www.fishlore.com/uv_sterilizer.htm

The thing to remember about UV sterilzers is that they only affect what passes through them, running one on a single tank is all but useless because there is no way to get all the water through the UV, at best you might slow parasites down. Running a sump system or multiple connected tanks on the other hand they are a great idea and can be very affective at stopping transfer between tanks.

I have heard suggestions that running a UV in a tank full term makes fish more susceptible to issues when you turn it because they are used to living in a sterile environment

I've heard the same thing. I know of an LFS that is reluctant to buy fish from UV sterilized tanks.

so i should only use it when the tank is sick? if i was to set up a stand with 6 tanks using 1 filter would 1 need to run that 24/7 or just when the fish get sick? thanks

Again it depends what you are trying to do. But UV is only limited use for treating sick fish (unless you put the sick through the UV :nilly: ). I'm getting the feeling you like the idea of UV and are trying to find a reason to have one, really UV should be left to people clearing green ponds and large scale setups thats have lots of fish entering and leaving.

For most people a quarantine tank would be far more effective.

Actually, let me rephrase that...For people with self control a quarantine tank would be far more effective. I got rid of my quarantine tank after I finally managed to sell the last of some 300+ bristlenoses out of it...

For people with self control a quarantine tank would be far more effective. I got rid of my quarantine tank after I finally managed to sell the last of some 300+ bristlenoses out of it...

so no need for self control now?

my friend has one on the outlet of a canister filter so it doesnt really get turned off is this a good or bad thing?

UV has limited penetration of water and needs to have the water very close to the light source. It is very dependant on treating clear water or the light will not penetrate. There is also a minimum contact period so the flow rate is important. There is also an optimumum operating temperature to work effectively.

I built one many years ago to reduce the creebies in salt water I was using to keep native marines and it is not easy to meet all the above requirements. The actual requirements will vary a bit depending on the make of the "sterilizer"

he got it to get rid of green water about a year ago but just left it going, would that affect the fishes immune system?

I doubt it.

Here's me making an educated guess based on how it works with other animals.

If you have a juvenile fish that is born and raised in a tank with a UV filter; it's immune system will be fine given its surroundings but considerably weaker as compared to a fish that was raised under normal circumstances. Most animals (humans included) inherit a certain amount of their immune from the parents (again - not sure how this works for egg laying species) and some of the immune develops within weeks to months to even years after birth.

yes some is inherited even in egg layers

the rest is from the environment they are brought up in

Hmmm perhaps ponds are different but I had a UV on the tank to help clear a whitespot outbreak then, when I had fish dying in the pond, I added it to the pond. The fish stopped dying (but that may have been coincidence) but I had a sudden outbreak of blanket weed that I had never seen in the pond before. It started to smother everything. I turned the UV off and the blanket weed died. :-?

This seems the opposite of what others are saying on using them on a tank :dunno:

I doubt it.

doubt what?

he got it to get rid of green water about a year ago but just left it going, would that affect the fishes immune system?

I doubt it.

That.

oh i just thought because everyone was saying if you leave it going it weakens the fishes immune system, it has been going for a year.

Because there are so many things working against it being effective. As I said earlier---I doubt it would sterilize anything.