Common goldfish (single tail) are not suitable for aquaria and certainly not for a tank of 30 litres. Fancy goldfish are suitable for aquaria but only of a significant size and Im talking about 100L for the 1st fish and then and additional 40L or so per fish on top of that. For a 30 litre tank you are really only looking at housing shrimp and snails and no fish.
Your best bet is to go to somewhere like B&Q / homebase and buy one of those plastic storage tubs thats at least 100 litres in size and move them in there, you will need to add the filter, maybe a heater set fairly low (around 20-22 degrees C) and an air pump to aerate the water. You will most likely have to do a Fish In Cycle which is pretty bad for the fish but not a lot you can do about that now. there will be some articles on cycling a tank on the forum.
You will have to do large daily water changes of at least 50% of tank volume to keep on top of the ammonia/nitrite build up as these are toxic to fish.
You can run it on a permanent basis but you need to take some of the media from the main filter to seed the smaller filter. If you seed the smaller filter and then turn it off the bacteria will die off without the water flowing over the sponges.
You should also be able to use any of the sponges you currently have in your main filter.