Actually you need a that trace of nitrogen from doodoo to keep the tank healthy.It is fairly complex. The nitrogen is absorbed by the algae.They keep the tank water full of oxygen. Oxygen breathing bacteria are the good ones that eat the bad germs.
There was never enough caca in my water to show in the many tests I have had run on my rainwater
It is perhaps worth noting that humans drank nothing but rain for a half million years both direct and from streams, up until our grand parents time.
Bottled water came to CC about 10 years ago. Many folks made jokes at the idea of tourists buying water when it was plenty and free for sky juice.
Now many locals buy it though most say rainwater tastes better. Rain is nice in the shower too.