There are internal ones and external ones.
I use a pondmaster 11w hooked inline to my fluval externally.
"sophie" wrote in message
news
In message , blue sky
writes
To risk sounding like a broken record...

Uv sterillisers have helped me combat algae problems when i didnt want to
use phosphate removers, unecessary chemicals that do more worse than good,
or extra fish, that add to stress on your bio filter if you have reached
the
limits of your tank...

do they all have to connect up to an external filter?

trust me....i have crystal clear water...better than any chemical would
do!
"nk" wrote in message
. ..
I use a product called algone.

www.algone.com

Been happy with it. Our water has high phosphates causing brown and
green
algae and this product really helps control it.

n



--
sophie