I'm a phycologist.  I have ...

I'm a phycologist.  I have over 25 years' experience in analysis and routine monitoring, and troubleshooting algal contamination. Often the best solutions are the simplest, but you may need to invest some time and money to achieve a level of control you need. This of course is all predicated on WHAT you are using the water for!

You potentially have several different types of algae in a rainwater holding tank. Given you have an algae issue, I assume the tanks are open and not covered. So here are the challenges you may have depending on what the tanks look like, the texture of the inner surface, how the water flows into them, depth, turbidity etc:

• If the water column is clear, you may have benthic algae, if not the tanks may be susceptible to surface blooms, but they may be algal mats lifting off the bottom
• If the inner surface of the tank is rough, you'll have mould, algae, fungi, and bacteria attached to the surface
• You may also have benthic algae growing along the bottom.
• If the turnover rate of water in the tanks is low, this will influence the population profile and may encourage cyanobacteria, if the turnover rate is moderate or sporadically flushing, you may have a more unpredictable population profile that changes after every flush event.

Physical removal techniques simply remove what you can physically see, and may improve the water column. However, that may not be where your problem is.  

Without know the size of the tanks, type of algae growing, use of the water, the risks associated (toxins, treatment downstream etc.), specific advice is difficult because rarely does one solution fix it all . But some basic questions and advice are as follows:

1. If the rainwater is flowing in from a pipe, can you cover the tank? If there's no light, the algae can't grow. But you may need some initial treatment and testing to know the water is safe both before and after.
2. With rainwater, especially with runoff, you'll have plenty of nutrients, but light is the one thing they all need.
3. Assess the risk. Get the water analysed for algae and associated toxins if any potentially toxic cyanobacteria are detected.
4. Gather your information; Is the water being used for stock, domestic use etc., and is there any evidence of illness, dermal, or mucosal irritation? Have there been stock deaths. If there is, physical removal is not enough.

There are probably a few more considerations here, but this should get you started. I have some info on my website. www.jarvishuntconsultancy.com.au