There is a wide variation avaible.
Most of them are external, and some are submersil.
And for sure they are not pretty to look at.
The task of UV-Light is, to kill algea-seet but be aware UV-Light do not kill string-algea-seat but they also kill a lot of benefital material such as bacterias, fish-eggs and even some seet from your pondplants.
UV-Lights need to be installed into your water-system. Mostly behind your filter.
Now what size UV-Light would you need?