1 U.V. or 2?

I have a 12,000G pond with two pumps going around 2500 GPH each in 2"
pipe into a large filter.

I installed 1 U.V. (36 Watt Tetra-UV3) about 5 days ago The U.V. is
rated to 4000 Flow and 8000 gallons.

I haven't noticed any significant difference in green water. I have
verified that it is working and seems to be an impressive unit.

Should I add another U.V. to the other pump as well?

How long does it take for algae to replicate? Is it possible the
algae is replicating faster than it can be zapped?

Or should I wait for two weeks to see how well it does?

Does the U.V. green water change happen "overnight" or will you see
little changes over time?

(Sorry for so many questions).

- Frustrated in U.V. land.