Oh well I missed the 60l tank bit, not sure how may gallons that is. I'd advise 30 gallons each to start, 20 minimum. Even for the next few days!
I would keep the filter with out touching the media or rinsing it/letting it get dry and do a 100% water change. Frankly do a new batch of conditioned water daily- no important bacteria lives in the water only on surfaces like gravel and filter media house bacteria colonies
Also a sponge filter is fine for a Betta or nano fish but the Goldies have a massive bioload and you can use a hang on back or even internal filter to meet their needs. Real poop factories, similar to a dog actually
What kind of pup did you get?