Water Tanks, page-10

No experience but from memory what I have read.

Clean gutters before first rain and allow first rain to run off, that gets dirt ect off the collecting roof surface.

Have a leaf/ litter strainer and keep it clean

There should not be any bacteria in the water tank, if you have it then sterilising with chlorine is required.

What you think is bacteria may just be algae, still not good but not as serious. Algae doesn't form/grow without light