Um, don't mean to be a pain, but a 30 litre tank is not big enough to house even a single goldfish...

For Fancy goldfish I'd say, as a bare minimum, maybe 60 litres for one and 100 litres for a pair. Common goldfish are really better off in a pond - you could maybe house a pair of babies in 200 litres, but they would outgrow a tank that size fairly quickly.

Sorry, if I had a spare decent sized tank I'd let you have it, but I don't

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