This is what I would do: re-home the goldfish and the swordtail. Goldfish need to be in a tank of 50 gallons at the very, very least considering they grow 5 inches at the very smallest, many of them grow over 10 inches! The swordtail needs at least 20 gallons as they grow to about 3 inches. The only thing a 17l is good for is dwarf shrimp. I'm sorry, I'm not trying to give you a hard time here but I'm only looking out for the best interest of your fish.
I also want to save you the heartache of fish illness and death.