A UV sterilizer will help kill and control anything that is suspended in the water column. I think if you are do that many water changes in a week then you may be stuck in a constant tank cycle.
What do you have in the tank right now?
What size is the tank again?
Do you have Po4 test kit?
Do you have a Kh, Mg, Ca and copper test kits?
I consider these the basic test for a reef along with Ph, salinity, amonnia, nitrite and nitrate.
So there are 10 basic test you need to do and find out where those peramiters are to start to figure out anything.
You could test for about 100 different things but IMO these are a must.
Can you get these peramiters for us?
and slow down on the water changes you may be going more harm than good doing that many all the time. If you don't have the test right now most LFS can do them for you and get the number from them for each test. Lets get down to the bottom of this.
there is a forum in the US if you want help you have to get these parameters before anyone will help it is a known thing on that board I guess.
Your corals could be doing badly for more than one reason too many water changes they don't like to be disturbed to much or have things change to dramatical all the time.
25% water changes 2-3 time a week could be a contributing factor to corals closing up and dieing. I don't know at this point need parameters.

Bill