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Baby chicks need very little room, but they grow really quickly. By about 6 weeks, they need about 2 ft per bird. Or at least, that's what I get mine. I don't know if tapping on the glass will affect their brains, but I can't imagine it would be good for their hearing. Or their stress levels. I don't think a chick under those circumstances would be very friendly.

 

If you're thinking about brooding chicks in a aquarium, please re-think. There's no airflow, the glass holds the heat, and there's not enough room to get out from under the heat when they need to. Find a big cardboard box, or better yet, two. Cut a chickie door between them and put the heat lamp in one box and the food and water in the other box.

A day old chick doesn't need much room, but a three week old chick does. When you start trying to crowd them you get pecking or cannibalism. And consider how long they'll need to be in the brooder before you can move them outside. If it's going to be six or eight weeks, you'll need at about two feet per bird---that really adds up!

 

I used a 40 gallon breeder tank (wider than tall) as a brooder last year, and it was fantastic! I did replace the glass top with hardware clothe for air flow though. The chicks were super friendly. You're right though, they couldn't stay in there long. I kept them in the aquarium for about a week before they went into my second brooder, which is a homemade hutch. My whole family loved the aquarium, I put it in the living room and we literally had chicken tv.

OP- I can't imagine any animal in a glass enclosure would like having the glass tapped on, so I definitely wouldn't go there. Good luck!
Nikki