So you have a goldfish in a 20 litre tank, if so they given you terrible info on goldifsh.
What type of goldfish do you have and how many.
The first common goldfish needs 20 gallons then 10 gallons for every other one added.
15 gallons for the first fancy goldfish then 10 gallons for every other one added.
Goldfish are massive waste producers so need double the filteration to tank size.
crap i mean 30 litre tank ><
And yeah we got 2 fancy goldfish in there. and i think there called minnow's the bloke sold us