Bleach
You really should do it no matter what other people say about bleach.
That's only when you want to kill your fish.
I rinse mine out with tank water, sometimes tap water, and then dry everything.
After that I just put it in the tank and take a small cup to fill up the filter so then it can suck up tank water. Then the filter can start running again
It's important to rinse out the filter every week or so to prevent too much gunk build up, and also the filter sponge or cartridge what ever you have in your filter, it should be rinsed out every week, because the water pushes the dirty stuff down and then comes up going through the sponge, charcoal, and bio beads.. but that's just an Aqua clear filter.
If you have one of those penguin filters, they are said to be much better

they have a bio wheel that helps reduce more waste and so on.. and the cartridge is really good.
You just have to rinse it out every week as well.