06-08-2012, 10:15 AM
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20g is the absolute minimum a single goldfish should be kept in, you should always plan on "the now" not the future, many reasons can come up that a bigger tank will not happen.
If you really want to get it then, daily checking of the water with a good liquid test kit such as API Master will need to be done, this will give you a clear picture of what is going on in the tank.
Filtration will be a bit of an issue as a 10g filter is not nearly enough for messy goldfish,
Temperature if you know exactly what goldfish it is, depending on type this can range from cold to tropical at 45 - 78F
Feeding would need to be kept to a minimum and regular vacuums of the tank would be needed to remove poop and other uneaten food waste.
At least one or more big water changes a week as well, this will be indicated by the level of Nitrates in the tank and whether or not any Ammonia or Nitrite are present. Daily if you notice any Ammonia or Nitrite.
In that size tank, I simply would not add anything else, the bioload from the goldfish alone is immense and anything else will likely make keeping the water parameters good very hard. Perhaps look at getting a 40g breeder tank off craiglist or look in local classified ads of papers. There is not a lot of difference in terms of the costs of going from a 20L to a 40g Breeder. HOWEVER, the benefit for the fish makes it well worth it.
If you look around, then there are deals on used tanks to be had, if money is an issue then secondhand provided the equipment is fully functional is one way to proceed.