Algae formation in water treatment process

I operate a ​water treatment ​plant for ​drinking water. ​
We use ​Birm® as a ​medium for the ​reduction of ​dissolved iron ​and manganese ​compounds from ​well water, ​followed by a ​sand and ​finally a ​carbon filter. ​

Our problem it ​that we ​have lots of ​algae in the ​product tank. ​

How do I stop ​the algae from ​forming? ​
Which ​substances and ​treatment ​methods do you ​use and what ​are your ​experiences? ​