The filter sponge is where the beneficial bacteria live which help to take ammonia, turn it into nitrite and then finally convert it into nitrate. These bacteria can also live in the substrate (gravel, sand, dirt etc). In a well established tank, which is what is sounds like you have, your main concerns will be the nitrates.
There's a lot of debate about changing filter material. You don't want to throw away your existing sponge because you're basically throwing away your bacteria with it. Whilst you may find much of your bacteria lives in the substrate, you don't want to get rid of any more bacteria than you can afford.
To clean your filter sponge, take it out, squeeze it a few times in some tank water to remove detritus and then replace it in your filter. As Trond alluded to, you may wish to do this once you notice the flow rate in your tank drop.
The main thing I would be concerned about
In a tank which is only 30 litres, you need to ensure you keep a good water chemistry balance. With less water, a small change in the environment can have much bigger consequences on the fish. A larger system is much more stable which is often why you see beginners with small tanks struggling to balance their tanks.
I would double check your reading on Nitrate to make sure it really is 0. Admittedly, the bio load in your tank is not high, but just double check anyway. Consistency is key and ensure that you do your weekly water change will help your fish thrive. Just ensure that in a lower volume system, the water you're putting in is as close to the parameters in your tank as possible to avoid big spikes in your water parameters.
I have seen 'no maintenance tanks' thrive. This is predicated on the idea of balance and normally only works in planted tanks. The plants in the tank will help to keep your water quality high anyway by using the nitrates. This means that there is not really a need to change the water as long as the plants get everything they need to grow. The balance is often a little tricky but it sounds as though this could be what is happening in your tank. Even a 'no maintenance tank' should be monitored closely though to ensure that no levels become harmful to your fish.