Broadly speaking algea wants to be stationary. You see it floating on the surface in stagnant water, or sometimes attached to rocks in moving water. Dilution is also a factor, but if at all possible you want to avoid taking from anywhere with significant algea even if the water is flowing. Prominent algea likely indicates high level of agricultural runoff (maybe fertilizer, maybe manure).
If you have to and you can afford to spend ~$30 buy Aquamira. It's mainly advertised for reducing the oder of stored water, but it's also commonly used by backpackers to purify water. Anecdotally I've used it to drink stagnant water (when no better options, of course) repeatedly with no issues, whereas I and others I know have gotten sick from clean moving water we put through a filter (Sawyer squeeze). Aquamira is just a way to make bleach on the spot in the right concentration.
If you can't afford Aquamira or don't have access to an outdoors store that sells it you can get the same effect with household bleach. It's crucial you buy bleach that only contains chlorine, not any scents or additives (for example to make it thicker and stick to walls). See https://www.cdc.gov/healthywater/emergency/making-water-safe...
After more research it seems you're partly right. Depending on the PH of the water and the type of cyanotoxins bleach might not be effective. It is effective in some cases, but not reliably.
If you have to and you can afford to spend ~$30 buy Aquamira. It's mainly advertised for reducing the oder of stored water, but it's also commonly used by backpackers to purify water. Anecdotally I've used it to drink stagnant water (when no better options, of course) repeatedly with no issues, whereas I and others I know have gotten sick from clean moving water we put through a filter (Sawyer squeeze). Aquamira is just a way to make bleach on the spot in the right concentration.
If you can't afford Aquamira or don't have access to an outdoors store that sells it you can get the same effect with household bleach. It's crucial you buy bleach that only contains chlorine, not any scents or additives (for example to make it thicker and stick to walls). See https://www.cdc.gov/healthywater/emergency/making-water-safe...