Under gravel filter?

I have a under gravel filter for my small tank do i need to clean it out or not because of the benifical bacteria

Eventually the gravel will become blocked up and will need cleaning.Do not use tap water as the chlorine in it will kill off the beneficial bacteria.You will need to use the water you take out of your tank to rinse the gravel.Save 50% of the water you take out to put back in then top up with de-chlorinated water,you can do this by leaving the water to stand for 48 hours it will help if you aerate it.

there is a page on caudata culture that tells you about fillters its in the articles section

Bacteria do not live on the undergravel plates, but live in the gravel.

However, unless you're using a powerhead, you will have a lot of 'gunk' buildup under the undergravel plates, which can cause bad water quality.

To clean gravel, get a siphon. It's the best way to clean.