We're talking about closed reservoirs, right?
Algae, to grow up, needs heat and light.
If you provide a closed tank reservoir, algae growing phenomena should be quite equal to 0.
Also, if you fed reservoir with potable water from public network, you should avoid any kind of undesidered growing.
Talking 'bout open reserves, you should comply with NFPA regulation with suction screens and "filters"
As indicated by Pete, algae should normally not be an issue with closed water tanks (unless the problem is coming from the water supply upstream your tank...).
If you have issues anyway, chemical treatment can be determined by a specialized company.
This has to be adapted to your conditions and considering possible corrosion issues in the pipes downstream (which would be undesirable).
It could be the same issue as for the presence of zebra mussels in some parts of North America (chemical treatment to consider).
We were faced with a similar problem caused by legislation in South Africa and tying in to potable water irrespective of the tank being closed. It was a possible hazard identified during a Hazardous Operability Study. An additional problem was that the water system fed a micro spray system requiring an extremely fine mist. The product reacted to water and light. Problem solved. We installed a dosing system to treat the water in the tank.
There are GRP Based Sectional/Panel Tanks rated to prevent algae growth by being 100% opaque and listed by well known third party agencies , WRAS certified and usage of SS 316 fittings provide storage of water with desirable quality.
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