I have a 20 gallon tank, we moved almost all the fish out of it gradually over the last 3 weeks and into our new 55 gallon tank. It has been going green ever since. we left two otos in there and our snail...then the snail died

we got 4 black rasboras and a
betta in there with the otos now.
ive been keeping the light off, but now the plants are starting to suffer. the little grassy stuff (cant remember what its called) is dying off. ive been doing 20-30% water changes a couple times a week, ive vacuumed the gravel, ive cleaned the filter, ive tried the particulate clumping stuff....
it just keeps.being.green! i cant even see the betta in there.
what else can i do? what am i doing wrong?
this is pretty green:
