Water Change, How Much?

I have been doing a weekly water change and a fortnightly filter clean which I find works best for me (and the fish!)  Each time I'm changing 25-30 litres of water.  My tank holds 170 litres, minus displacement for plants and ornaments
Is this enough or too much?

The once every 2 weeks filter clean is fine. The brown goo that builds up on the media is a "nitrate factory" - all the uneaten fish food, fish poo, bits of dead plant which make up the goo release ammonia which is turned into nitrate by the filter bacteria. Washing the goo off every couple of weeks helps keep nitrate low.