paghat wrote:

...1) Chlorine kills soil microbes without which plants cannot access
nutrients in the soil.

...5) As tropical fish hobbyists have long known, chlorine in tap water
kills fish...

...7) There are better methods of treatment, but not cheaper ones...

Hydrogen peroxide can be used. It's probably not cheaper than chlorine,
but it's not generally considered as an option. At concentrations that
kill algae, it may also have an effect on the microbes. Fish can survive
in moderate hydrogen peroxide concentrations (not sure about tropical
fish). Also, the peroxide does not dissipate in the same way as
chlorine. The peroxide is removed through oxidation reactions which is
how it kills things, otherwise it's fairly stable. The by-product of the
oxidation is water.

Another alternative is oxygenation. I have heard that farm ponds with
algae problems can be treated by simply bubbling air through the water.
I have not tried this, so it's hearsay.

I think the original post was about algae in a water bucket. If a
somewhat larger water storage system was used, fish might be an option
to keep the algae down. Once again, it may not be cheaper, and there's a
maintenance issue involving feeding the fish and storing them over the
winter.





...It is also puzzling to me that people can take such a strong dislike
to algae. Floating algae maybe, it ruins visibility, but algae is not
the devil that chemicals can be.

If you're watering through a drip system, algae can clog the emitters.
If your water barrel has a lot of organic solids in it (including algae)
it can breed mosquitoes unless it's covered. Otherwise, the algae is
strictly a cosmetic problem. For drip systems, the problem can be solved
through filtration rather than algicide, although filter maintenance is
then an issue.