Myself I don't use a uv sterilizer but I am shoping for one that can help with algea that is in the water colume, kills parisites and also good and bad bacteria if you have flatworms it will kill any that pass through the UV unit and that goes for atapsia and mojano spors also. I siphon my sand as a part of my maintance so 7 to 14 days, you may loss some good stuff but it will come back. It will stop the sand from caking and suficating the layer below which is a good thing your biological filter is throughout your sand bed and if it suficates you nitrites can rocket as the biological matter decomposes so you phosfates will also go up and you get a algea bloom of some sort and that can be part of what is happening to you when you clean the top layer you alow the biological filtering to become reistablish but then the sand cakes again and you are back where you started. changing the water flow so it disturbs the sand a bit helps to keep it from caking. You want very little water droplets inbetweenthe sand particals if it cakes these water particals are not there and the sand bed suficates. I think I'll make a educated guess that you don't have to much algea on the rocks but more so on the sand where the decomposing of the sandbed is taking place.

To fix things you can't just mix it up siphon up the red algea and after that siphon the sand bed get as much of the ditris out as you can. Scrape the algea off your glass below the sand bed level, it will come back a bit in time but just do a good cleaning. You will have to make siphoning a part of the reguler cleaning.

It only takes a bit of work, try it it will work, the sand sifting star will like it; also the gobie will too.

I hope this helps,
Bill