I was bought a pair of red and white goldfish for Christmas. My husband spent two weeks setting up their tank prior to introducing them to it and we do regular PH and nitrite tests and once-a-week 1/3 water changes and everything is, excuse the pun, going swimmingly!! I am just a little curious at the swimming patterns that are developing between the pair, they seem to get on very well together, are very lively, bright and growing by the minute, so I am not overly concerned, just looking to expand my understanding of their behaviour. They seem strong swimmers and appear to enjoy taking turns swimming in the fast flowing water from the top of the filter tube.
Sometimes the smaller of the two likes to swim with his head underneath the tummy of the other and they are have started looping around one another as they swim, it is not in an aggressive manner whatsoever, rather more playful. Are they simply playing or have I got a boy and a girl?!!
Angela, the smaller fish is most likely male. He may develop breeding stars on his gill plates and the front edge of the pectoral fins. The other fish is likely female. Remember you need to test for ammonia and nitrates as well as Nitrites. PH is also useful to test.