Hi, gd morning..
Sad to hear about your lost..
Firstly, if you could list out your fishes and give a description of how they died? -Gasping?, swimming erratically, rubbing against the rock?, not eating?
Secondly,
What kind of parasite may i ask you referring to?. (Common Ich, isopods, velvet etc.)
Assuming all your water parameters are in place.
There are only a handful of options you can try.
Running a UV Sterilizer is a good start, but too high a wattage can also kill benefical bacteria in your tank, how about sharing with us your UV wattage?
If we are talking about parasite?.. microscopic or big parasites?..
Does your water have enough gaseous exchange?.
One way to promote gaseous exchange can be agitating your water surface with a wavemaker/powerhead..
Also can you recall whats the last fish that you add?.. Did it promote or spread the symptoms to other fish?