Potable water from rainwater - questions, common issues, & fixes?

When I first learned about rainwater catchment systems (metal roof --> big gutter --> big holding tank --> some kind of filtration --> pressure pump --> maybe more filtration --> faucets, showers, toilet, laundry machine, etc.) I figured "cool, those are the parts, put them together correctly and they just work."

Someone on here (John C. Daley, I think?) has said you don't even need filtration. I accept that this person has not needed it, but I wonder if it depends on what's in your local rainwater, also perhaps on the design of certain parts of the system. (How localized are quality differences in rainwater anyway?)

Last night I watched a video by a guy who built an off-grid tinyhouse in Bulgaria or Belarus (some eastern European country whose name started with B, sorry I forget), and he said he used rainwater catchment exclusively for the first few years but got sick of algae taste and smell in his water, and had noticed the walls of his bathroom even started to turn green, so he switched to a well.

How common is the issue he described? What causes it? Did he simply do something wrong in his installation?

I definitely get a lot of algae in my rain barrels, but they are outside and not UV protected so I kinda expected that.

What else is there to say about purity/potability from rainwater catchment?

(Sorry if I'm repeating previous thread/s)

Ned, fair questions!

Someone on here (John C. Daley, I think?) has said you don't even need filtration. I accept that this person has not needed it, but I wonder if it depends on what's in your local rainwater, also perhaps on the design of certain parts of the system. (How localized are quality differences in rainwater anyway?)

Last night I watched a video by a guy who built an off-grid tinyhouse in Bulgaria or Belarus (some eastern European country whose name started with B, sorry I forget), and he said he used rainwater catchment exclusively for the first few years but got sick of algae taste and smell in his water, and had noticed the walls of his bathroom even started to turn green, so he switched to a well.

FILTRATION
- First flush systems are smart to reduce, dust, bird poo, leaves and twigs.
- cartridge filtration can be eliminated by having a large, 5000 gallon storage tank
- large tanks, just like municipal dams give time for settlement of suspended solids,
- time for interaction between the water surface and oxygen which deals with pathogens.
BULGARIAN RAINWATER COLLECTION SYSTEM
I found that video,
- Everything he says I have seen before, and can obviously happen if the design is awry
- at this stage I am only guessing, but my first question would be about the volume of water he had in the storage tank.
- algae in an underground tank with no sunlight is odd

More information would be great.
I have written to him.

Ned Harr wrote:metal roof --> big gutter --> >>>>initial diverter to flush all the algae fertilizer like bird poop etc. etc. to a different location before capture>>>> big holding tank --> ...

I don't claim to be an expert. From what I've read, this seems to be an important component in the system.

Edit: John C beat me to it.

Access to oxygen is also an important part of self cleaning.
I wonder if an underground tank would have access to oxygen readily, in the same way a surface mounted tank may.
I imagine a surface tank will have a surface available to the top for aeration to take place.
A submerged tank may have no surface area available, just an overflow somewhere.