Its quite hard to tell the sex of fancy goldfish, with slim-type goldfish like comets and commons the females will become noticeably fatter/rounder as they sexually mature and start to carry eggs- i think (although i could be wrong) with fancy goldfish like oranda's, males develop a little bump in dorsal fin at the front when they sexually mature. Goldfish take many years though to sexually mature, usually it takes about 3-5 years for goldfish to sexually mature depending on the variety.
To breed fancy goldfish in an aquarium you need to have a densely planted area in the tank (preferably with plants which have lots of small leaves), however this can be difficult as goldfish also eat plants. Even if you get the goldfish to spawn, they may still eat their own eggs if they find them in the planting after they have finished spawning.
In ponds, it is a lot easier to breed goldfish, since due to the large body of water and plenty of space for lots of planting and the seasonal effects on the water temperature, spawning is a lot more likely to be successful in a pond. Fancy goldfish like oranda's are not really outdoor fish though since such fancy goldfish cannot be kept outside during the winter since they cannot tolerate freezing temps like non-fancy varieties of goldfish can.
Still, none the less, i have still seen a few cases of people breeding fancy goldfish in aquariums successfully, although your best bets for breeding them is to get a large tank (say 80gallons or more long) and have quite a few mature oranda's of both genders (since the more goldfish you have the more likely they are to create a mass spawning session), plenty of planting in certain area's of the tank, and to carefully change the temp and periods of time you do the water changes and the size of them etc to help mimmick the conditions which take place outdoors during the end of winter and start of spring which get goldfish in the mood for breeding
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