You know that those birds need opportunities to do long flights? Also, they need more spacious cages than are sold, giving them more chance to be active during the day. This is always underestimated. I did the same mistake once and ended up having a bird who pulled out feathers from his tail and neither really spoke nor really sang. Cockatiels are just too pretty and long-living to be cared for poorly. Just like humans they can look pretty normal while being broken on the inside.
The cage extends to twice the width shown in the photo and fits three perches, a ladder and three small to medium sized toys while leaving enough room to freely turn anywhere. I thought that'll be large enough for one bird.
I take him out twice a day to roam around the apartment, which, based on the reading I did, should be enough to provide needed excercise and interaction.
I really hope I am not creating a sad bird O.o Thanks for voicing your concern. I'll be sure to have a careful eye on his happiness.