Hi negusma,

in your 35 l tank you can keep one male fighter and you can add a few tetra fish or rasboras.
Initially I had our Barbarossa ( siamese fighter ) with 10 neon tetras, 2 corydoras and one Yoyo loach. ( it was a mistake to have a Yoyo loach with the fighter, as when Yoyo grew up, he nipped at the fighters fins.)
So I moved the fighter into a 20 l tank he is in there now with two corydora panda and 6 rasboras, very happy. And yes I use filtration and aeration.

Please don't get a goldfish as the tank is too small for one, you need minimum 10 gal, as already posted several times before!
Also goldies like to have company, which you CANNOT provide at all at this stage. Better save your money to get an appropriate tank and equipment and then get your goldies.