There is a device called a Python that's a very long plastic tube that you can attach to a sink faucet. It can be used to both drain a big tank like yours (into the sink) and to refill it. Didn't you say you live in Greece? I'm hoping it's available to you there. Here's a link that describes it. It's available at all major pet stores and on-line suppliers around here.
If you can't get a Python, you can get a simple syphon (let us know if you're not familiar with this device), drain the water a few gallons at a time into a bucket, then either refill the tank with the bucket or run a garden hose in from outside to fill it.
Last edited by SpotsMama on Fri Jul 13, 2007 1:21 am, edited 1 time in total.
Even with a wonderful filter you'll have to change the water from time to time. With a good filter you just won't have to change it so often. I think most people with good filters and tanks that are right-sized for their turtles do partial water changes every few weeks and change most of the water every month or six weeks.
With even a good filter, a byproduct of turtle waste breakdown (nitrate) will build up and it stinks. A good biofilter will neutralize harmful waste products like ammonia but not the nitrate. Also, as nitrate builds up algae will probably start to grow. A little algae is fine but too much and you won't be able to see your turtle in the tank!
The only practical way to get rid of the nitrate is to drain some water.