There are many many places where you can feed fish in Bangkok, and each time you do it, you are helping make some fish very fat and lazy. Never mind, it's fun to watch them go berserk in a wild thrashing of tails.
These are in the Chao Praya river, the river running through the center of Bangkok, the feeding area is near to Ratchawong pier. This is the pier you stop at when going to China Town, so you can combine your fish feeding with a visit to Bangkok's China town.
The cost of the fish food was 10 Baht, the location is on the map below. When I talked to the seller, he told me these are just babies, the really big fish are further up the river at Tha Tien pier, but they look huge to me.