Not aggressive per-se but sometimes they get a little amorous. :o
ON THE NOTE OF COMMON
Goldfish. Hang in here, what you're about to hear isn't pretty.
-Common
Goldfish max out at between a foot and 2 feet long adult length; the likelihood is that your goldie will likely grow longer than the standard-sized 10g. Not saying this will happen (because of stunting) but I have seen 2 6-inch comets in a 15g which is way overkill.
-Common
Goldfish need about 80g each.
-Their lack of a stomach makes them one of the messiest fish inch for inch and therefore they require double the filtration in gallons per hour (GPH) of an average tropical setup.
-Goldfish are social fish and need to be kept in groups of 3.
-While Fancies actually thrive at warmer temperatures of about 70-74 , commons are true coldwater fish and are not suited to tropical fish tanks. The ability of water do hold oxygen goes down at the heat goes up and commons need lots of oxygen and therefore colder water.
-Commons can live up to 40 years under proper care, usually around 20 or 30.
Don't get me wrong, I will tell you firsthand that
Goldfish are one of the hardiest, funniest and smartest fish out there but a large tank (or in the case of common
Goldfish a pond) is essential to successful
Goldfish care. I am also concerned that the goldie will pollute the tank more than what your filter will handle, exposing the other fish to high levels of ammonia.
Building a pond can be a fun spring project to spice up your backyard but until you gain proficiency at running the 10g I suggest you either give the
Goldfish back to the pet store or rehome him with a friend who has a pond.
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