Pond Usage
First and foremost, if you intend to use captured rain water in, near, or around a pond, choose a bleach alternative for algae control or insecticide. Bleach can be damaging to goldfish, koi, and other common pond fish. If you are intending to use bleach to control algae in your rain barrel, consider purchasing pond water algae control chemicals which are intended for use in water containing fish.
Alternative Algae Control
Bleach is often used in rain barrels for algae control. Algae is an especially problematic in rain barrels that do not have lids, or are transparent. As algae is a photosynthetic organism, meaning it gets its energy from the sun, it requires sunlight to grow and reproduce. Rather than chemically treating your water, consider covering your rain barrel or shielding it from the sun.
Washing
For barrels already containing algae growth, bleach may be used in a dilute solution, meaning less than 5 percent bleach, to clean out the barrel before rinsing it thoroughly with clean water. Algae can accumulate on the sides of a rain barrel and be reintroduced to clean water when your barrel is refilled. Be sure to thoroughly rinse out your barrel prior to replacing it under your downspout any time any chemical cleaner or solvent has been used.
Insecticide
If your rain barrel is uncovered, you will be facing an insect problem. Mosquitoes need water to lay their eggs and live out the first stage of their life cycle. Mosquitoes, as well as other water-borne insects, can be more than just a nuisance at your barbecue, they can carry disease. Adding a mosquito dunk, or small amount of olive oil to your rain barrel can help drown any insect larvae hatching there and keep the adults around your house to a minimum.