What is this?

I believe the smaller chamber to the right is the electrical ballast. Aqua UV is a good brand as long at the wattage is appropriate to the size of your tank. Their website has a fair amount of info on there as well.

 

I run a 57 watt aqua uv on my 120. The benefits are debatable...their is a group that says they are useless and a group that says they are beneficial. I personally love mine...i had a coralife uv that broke after a few months of use...in the three months i took to save money to get a aqua uv i noticed more algae and the water looked less clear. Two days after adding the aqua uv my water cleared and a month later the algae diminished considerably. If powerful enough and water flow is slow enough for contact time with the light they will kill free floating ich parasites reportedly. If you run one and are feeding corals live zooplanktons and phytoplanktons i would suggest shutting the uv off for 30 minutes to an hour so the nutritional value of the live foods isn't depleted and killed off through contact with the uv.

As for maintenance...clean the quartz (glass sleeve) once a month or so. Wear gloves to prevent fingerprints on the quartz when cleaning it. The uv bulb is recommended to be replaced once every 6-12 months...the earlier the better to ensure quality water sterilization. And check the hose connections every once in awhile to check for leaks.

If you have one you can try it by plumbing it in to your tank plumbing...which is very easy...and test it out and form your opinion on it. I think they are beneficial...you may not...its just a matter opinion...my opinion is that they are beneficial and added security for your livestock...probably not entirely necessary and they are an added cost for operating ($60 for the bulb+electrical operating cost per year) but i think that is minimal if i can use it to keep the investment i have made in livestock healthy...