I think they have duckweed in there, which grows like crazy from what i've read. So maybe they have not taken out what's not needed for space to remain at the surface. Ask them!
The algae is what is making it look so bad,
though algae isn't unhealthy to be in the tank, doesn't it still cause ammonia to rise as it grows? If they want the pretty green, they could paint the outside of it (safe paint) or just get some moss.
They need bigger homes!
"Life is an adventure full of beautiful things, and a treasure worth more than gold."