Looking for free-floating plant species

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While the local library has a lot of books about microalgae and
surface bound macroalgae, planktonic macroalgae/plants are – if at
all – only mentioned in foot notes.

I'm looking for plant(including algae) species that are at least 0.4mm
large and tolerate the following habitat:

- free-floating, completely submerged
- 10-30 °C (usually: 16-23 °C)
- 5-8 pH (usually: 6.5-7.5 pH)

Salinity (in-)toleration isn't an issue.

Thanks for any hints/ideas.

Kio Boskden