HELP! My fish got stuck in the filter today, and I only bought them this morning.

I have a Juwel Rekord 60 tank, and the filter is built in. It has slits in the side and at the front where the water goes in and comes out.
I was watching my fish (I have 6x Neon tetras and 6x Zebra danios), and then I noticed that there were only 5 Zebra Danios in the tank. One of them must have swam into the filter somehow, because when I looked there he was swimming about inside the filter.
He was OK, but we had to take all the sponges and everything out of the filter to be able to get him back into the main tank. It took us 15 minutes. Luckily he seemed to be swimming fine back with his other friends in the tank.
PLEASE. How can I stop this from happening again?
Thanks for any suggestions.
Gwenz