If this is a legitimate request, I think that you need to redefine the problem. In my experience there is no such thing as "all video codecs", as there are many many special ones that are specific to one camera or one video device, and there are even some pieces of malware disguised as codecs to try to trick you into installing them to play some questionalbe piece of video. And if you think that you have "all video codecs" wait until tomorrow and look around, you'll likely find someone posting something with yet another off-the-wall codec that he wants you to install. My advice would be to install a few major codecs and when you start getting messages that they are not enough, think twice about if you really want to watch the video.

I don't tend to install anything in Knoppix, as it has a greater chance of breaking things that when installing to a true hard disk targeted Linux distro. However, if you really want to install something, you might look at VLC by VideoLan. I believe it has all of it's codecs self-contained, and if there is any codec that it doesn't have you likely don't really need it.