Is this a 20 litre or 20 long? Either way both are not suitable for any type of goldfish. Goldfish get large, are very messy and need space.

Cycling, does not just mean leaving your tank to sit for a week. Its about adding a source of ammonia to the water and letting the filter bacteria build up. This can take over a month if you do it fishless.

For goldfish you are really going to need a canister filter. Also you never chage the bio media, only the mech media.

You should be doing large waterchanges weekly for goldfish

Goldfish do NOT need a heater. Warm water can make them easily get sick

Normal goldfish need about 30 gallons PER FISH with a min tank size of 40-60 gallons. Fancy goldfish need about 20 gallons PER FISH with a min tank size of 40 gallons.

If this is a 20 litre tank, get a heater and get a betta.