This. Trees (especially large ones) are a pain to irrigate properly, might not be drought-resistant, grow very slowly until they reach their full potential at removing CO2, interfere with infrastructure that we humans are used to (piping, electricity, telco), roots break up pavements, branches can be a hazard after storms, fruit might attract rats, ...

I'm very much pro trees (despite what I've listed in the first paragraph), but I'm sure there are places in cities where you can't plant trees but could put up algae tanks.

If you understand German (specifically Austrian dialect) you might like this podcast episode about challenges and methods to overcome them in the context of greenery in the city of Graz:

Simple Smart Buildings: Bäume in der Stadt

Webseite der Episode: https://podcasted3e6b.podigee.io/153-baume-in-der-stadt

Mediendatei: https://audio.podigee-cdn.net/1742586-m-9ecab280e580cd07f75c83ed9379b970.mp3?source=feed

TL;DL of this episode: it's not as simple as "just plant more trees".

Yes. Algae is better in absorbing co2 than tree, but tree is important as a shade and creating a cooling effect for the surrounding. Both is important for different thing and combine it you get the best of both world, especially in a lot of urban area where planting big tree isn't possible

Like walls of high-rises.

And trees that are planted in cities are not seeded. They are grown in a forestry until they reach a certain height. And then dug up with machines transported with machines and then planted with machines. The CO2 produced to plant a single tree also takes quite a while to be absorbed by that tree.

Who cares? You can sell these tanks for a better profit than trees.

What you can do is take all the trees and put them in a tree museum and charge the people a dollar and a half to see them.

That's where youre wrong.

If we put the algae in the oceans, then sink all of our cities underwater, all of our problems will be solved.