If you are really into paying a huge amount of money to watch depressed animals licking their genitals behind glass, you can always take the train and pay a visit the Planckendael Zoo. If you’re afraid of trains, a cheapass and/or prefer animals without real genitals we can highly recommend a visit to the Public Aquarium of Brussels.
The aquarium is a non-profit organisation run by volunteers who make their own website - which is even nominated for a ZooSite Award! - and clean all the fish poo themselves with a tiny vacuum cleaner. Not only does the entrance look like an African shack, they also have fish and amphibians from all over the world. Some of them are really ugly which is of course very funny.
They also have a souvenir shop where you can buy mugs with goldfish on them for only 3 euros. If you really liked your visit, you can even adopt a fish tank. They will put your name and a mugshot next to it so you can make your momma proud.
Avenue Emile Bossaert 27