Help with 1500 gallon fountain and Algae

Hi,

I have a large fountain in my front yard around 1500 gallons that is a constant algae pond. The fountain does not have a traditional filter but merely a pump to generate water flow. I have tried using 1 inch Trichlor tabs and Polyquat Algecide to keep it clear (no fish or animals in the fountain) but that does not seem to work. The Trichlor tabs are bleaching the stone in places and I'm about to give up on using them as they don't seem to do anything.

My next step is to consider adding shock to the fountain on a weekly basis just like my 25,000 gallon pool.

If I use calcium hypo shock, there is no natural stabilizer source so the chlorine will not last (fountain receives several hours of direct sunlight and always has leaves falling in it).

I was considering using dichlor shock since it contains stabilizer which should make the chlorine last longer.

Ultimately I plan to drain out the fountain every 2-4 months for a good cleaning but the less algae buildup, the better. Certainly less scrubbing for me. Any suggestions.