Aquarium Tank
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How many goldfish should you have in a 30 litre aquarium tank? And how many of those tiny little ones?
The little ones are coldwater fish too, you always see them in the tanks with the goldfish, don't know the name.
A 30l tank is too small for even a single goldfish. A single fancy goldfish needs a minimum of 38l (~10 US gallons), and an additional 38l is necessary for each additional fancy goldfish. The single tail common, comet or shubunkin goldfish need about 190l per fish (these are actually more suitable for ponds). Goldfish produce massive amounts of waste and because of this, require very strong filters, frequent chemistry testing and frequent partial water changes.
Most likely, the small fish to which you refer are white cloud minnows or silversides. Only experienced goldfish hobbyists should keep these with goldfish in large tanks. The minnows are schooling fish and care must taken when keeping them with goldfish as goldfish may eat these fish.
Your 30l tank would accommodate a shoal of 6-8 white cloud minnows. It is too small, however, for even a single goldfish
Community Fish Tank: 20 Gallon Fresh Water Aquarium