There are several reasons why this could be happening, so try checking the following:
- Did you shorten or remove the lift tube? A shorter lift tube does not have as much suction pulling bubbles up the center column, so some air may escape.
- Is the air stone crooked inside the sponge filter? To make it hang straighter, you may need to shorten the tubing attaching the air stone to the bullseye.
- Is the air pressure from the air pump too strong? If a bunch of air is forced into the sponge filter, excess bubbles may leak out the sides.
For more information on sponge filters, read our full article here.