Advice needed on UV filters.

[poppy]

I have a 6' 300 litre tank with mainly peacocks. Originally I was running an Aqua One Nautilus 1100 and a Sunsun  HW303A. Regular water changes and replacement of the filter wool kept the tank clean.  After the Nautilus developed a leak (o-ring) I replaced it with a Sunsun HW-303B with a UV light.  The UV was promoted as the next step or additional water purification system.  I added the media from the Nautilus and what was needed from the supplied Sunsun media to fill the trays.  Within a week I had an outbreak of stringy green algae. The HW-303A and HW-303B are rated at 1400L/H so it would at worst be turning the water over three times per hour.

I was under the impression the UV would provide extra protection from algae blooms like this.  I have since turned off the UV but I am still having problems with algae in the tank. The walls of the tank seem to get a heavier lining of algae than I have ever had before. This week I have deep vacuumed the substrate. I didn't clean the filters as they still appear to have good flow.  I have now removed all of the rocks to dry in the sun and kill off the algae.

When cleaning filters I fill a 30 litre tub with water siphoned from the aquarium. The sponges and media are rinsed in the tub and filter wool is replaced.  Only one filter is cleaned at a time.  Water is tested regularly and show no adverse results.  To verify the accuracy of my water tests I have the water tested at my LFS evey 6 - 8 months.  They always come back with the same results I get. 

What am I missing or doing wrong.

Edited March 24 by poppy
correct detail

[PlecoSam]

In my opinion most hobby grade UV systems are useless and probably just snake oil.  The UV systems provided in canister filters are simply ticking the box providing UV, in reality they do nothing.   

You are doing nothing wrong except providing excess light promoting algae growth.

Edited March 24 by PlecoSam