Algae in water tanks

No idea what is classed as biological anti freeze, but almost anything that works as such would not be healthy to use water from, even for plants. Despite all your cleaning, I strongly suspect that there's still a lot of residue caught in tiny crevices. This is not scientific, just a strong hunch. Best advice is to contact some sort of local health agency for further info, or perhaps a gov't agricultural department for advice.

I don't know what biological anti-freeze is either, unless it is another name for the "safer" version of anti-freeze using propylene glycol rather than ethylene glycol.

If you Google "pink algae" you will find plenty of information about it (it is actually a bacterial colony rather than a true algae.) It can thrive in the sort of environment provided by the inside of this sort of container, although is more of a problem in swimming pools because it is unsightly and also slimy.

Providing there was no solid residue in the tank the contents should have been completely removed by your flushing regime. If it were me I'd probably drain and it and refill once more and then stop worrying about it (I suspect not many of us examine the insides of our water butts and would probably be fairly dismayed if we did!)