Hiya, and thanks for the query.
The curse of the fairground fish! I have had similar problems
.
As you say, a small tank for goldfish is a no-no, and will more than likely lead to illness from mucky water and overcrowding.
You mention a 35 litre tank. While that is way too small for goldfish, it would be ideal for a single male betta. They come in all sorts of lovely colours, have beautiful finnage and can be trained to follow your fingers and eat from your hands (well mine does that anyway).
It is a tropical fish, not coldwater like goldies, but to be honest the only difference I have found in looking after coldwater or tropical fish iss that the tropicals need a heater in the tank.
Both types of fish need a filter, will need weekly water changes etc.
If you can get a tank a bit bigger, then there are several tropical fish that you could have in a moderate sized tank. Have a look in the Library section on the forum for some info and ask any questiobs you may have.
Good luck!