It’s expensive and has only the advantage of catching CO2
It doesn’t even do that well. Algae have short lifespans and when they decompose, the CO2 will go right back into the atmosphere. It’s the same reason you can’t reasonably capture CO2 with small plants like grasses, nor does the carbon inside you count as captured. The reason trees “capture CO2” is because trees live for a long time and wood decomposes very slowly, and therefore keep its carbon locked in the wood for a long time. The point of capturing carbon is you take it out of circulation for as long as possible.
There are ways to have algae capture carbon, but they are fairly involved (read: very expensive) processes whose scalability is still uncertain. Certainly not a tank in the street.