Goldfish reproduction is much easier in large tanks or ponds. Alternatively, a good volume breeding tank can give fairly good results. For more information, please go to the goldfish breeding page.
In principle, it is best to avoid putting slim-bodied goldfish (elongated shape, e.g. common, comet, shubukin...), which are faster and more playful fish, with fancy 'egg-shaped' goldfish, which are slower and more placid. This can cause issues with constant biting of the fins of the slower fishes in addition to unequal access to food.
Recognizing male from female goldfish is quite tricky. The main difference is that males quite often develop characteristic breeding tubercles (white pimples) on the back of the head (gill covers) and pectoral fins in early spring or summer (just a few small dots in these areas but not all over the body as would be the case with white spot disease). The female goldfish will have a rounder, thicker body shape and may also have a bulge on one side as breeding season approaches.
Under normal circumstances, when the biological filtration is mature and the filter functioning correctly, beneficial bacteria in the filter will utilise ammonia as an energy source and turn it into nitrite. Other bacteria then use the nitrite and produce nitrate, which you will remove by changing some of the water each week.
It's very simple. Quick and easy to use tests are available in pet stores.
Temperature is one of the many parameters to take into account: bacteria multiply less quickly in cold water (they can even stop multiplying in very cold water).
In order to make the nitrogen cycle in a new aquarium significantly easier and faster, it is possible to take a piece of the filter media from an already established aquarium (mature filter) and place it in the new filter compartment or squeeze out the brown juice rich in bacteria - making sure of course that this aquarium has not experienced any diseases recently - (note: using 'aged water' is also possible but it is certainly less effective). However, make sure that the filter media to be transferred is transported in a bag containing only water taken from the original aquarium. Lastly, you may also add commercial bacterial activators (nitrifying bacteria or filter boosters).
Nitrates are only harmful to goldfish in high doses (long-term swimbladder disorders, slower growth) but small amounts can be enough to feed a large amount of algae. Nitrate is the end product of the nitrogen cycle and can be significantly reduced in two ways: by making partial water changes regularly (once a week or every two weeks) and by planting the aquarium (nitrates are partially absorbed by the plants, which they use as fertilizer).
If you using the fish-less cycling method (more info here), not necessarily: you may wait until both ammonia and nitrites have dropped to zero and nitrates have risen before doing your first water change.
But if you already have fish in your tank, yes, do partial regular water changes. Performing regular water changes is crucial to the health of the fish and, in any case, beneficial bacteria colonize surfaces like gravel, filter media and plants, so this will not stop the cycling of the tank. Always use a quality water conditioner (dechlorinator) when doing water changes.
Water and water chemistry
Goldfish cannot live in untreated water straight from the tap. This is because tap water contains chemicals (chlorine and chloramines) that are bad for your fish's health. Chlorine and chloramines can also kill all of the beneficial bacteria in your fish tank. To remove chlorine from tap water, use a good quality water conditioner (available in pet stores). More details on how to make tap water safe for fish here.
The pH measures the acidity or on the contrary the alkalinity of the water. It is measured using a scale from 0 to 14. The number 7 corresponds to a neutral solution. Values above 7 are basic or alkaline; values below 7 are acidic. The pH reading is logarithmic: a change of 1pH corresponds to a 10-fold increase in acidity or basicity. Let's take a pH of 5: it is 10 times more acidic than a pH of 6 but 10 times less acidic than a pH of 4. Similarly, a pH of 8 is 10 times more basic than a pH of 7 and 100 times more basic than a pH of 6.
For fishkeeping of course, extremes are meaningless: most fish and plants prefer a pH between 6.0 and 8.0, and only a few species will require more acidic or, on the contrary, more alkaline values. Goldfish are adaptable but prefer a neutral to fairly alkaline pH (7.0 to 8.0 ideally, acidic water is not recommended). The pH is commonly tested using small test kits available in pet shops but a digital pH meter can also be purchased.
Whether you need to regurlarly test the pH will largely depend on the stability (or rather the instability) of the pH in your tank.
If your water is soft or seems to progressively get acidic, or if the pH is unstable, you will need to test your water regularly. The tests should be carried out at the same time of day, preferably in the afternoon, because during the day, as plants produce oxygen and use carbon dioxide, the pH may increase slightly, whereas at night the phenomenon is reversed, as plants and fish consume oxygen and release carbon dioxide, and the pH may then decrease slightly. This simple precaution will prevent misinterpretation. If you suspect that the pH level fluctuates significantly during the day, the pH should be tested twice, in the morning and in the evening, to determine the exact amplitude: an unstable pH level will always require urgent action to be taken. The first measure is to first make a partial change in the water: by replacing the old water with new water, you will add a buffer effect (these are dissolved mineral substances that would have been gradually depleted in your tank and whose property is to provide a certain resistance to pH fluctuations).
For those whose water is hard water, on the other hand, there is little chance of the pH changing: hard water contains large quantities of these dissolved substances that slow down pH changes, and regular water changes will suffice to "recharge" it with buffer substance.
Goldfish are much better than most other species to cope with a rather inadequate pH. However, care should be taken to provide them with water that is compatible with their well-being. A pH that is much too low (usually in a tank that is overcrowded and overloaded with organic waste) can cause acidosis: the symptoms (which are not exclusive, hence the point of testing) are usually apathy and refusal to feed, over-secretion of mucus, fins where blood veins appear. A very high pH can cause, but it is much rarer, an alkalosis - but above all increases the toxicity of ammonia: the fish remains on the surface unable to breathe properly and secrete a lot of mucus. Beyond that, sudden pH variations are the main danger and may put fish under great stress.
Generally not recommended: rainwater is sometimes used but introduces a risk of chemical pollution (and even more so if it is poorly recovered) so we would not advise it. Moreover, it should be systematically mixed with conditioned tap water or remineralized with commercial products because its hardness is almost nil (which causes very dangerous pH variations): for goldfish, which appreciate relatively hard water, from neutral to alkaline pH, using rainwater is therefore of little interest.
To be avoided: distilled (or osmosed) water used pure would be totally unfit for life because its hardness is nil: it must absolutely be remineralized according to the hardness sought.
Water from domestic water purifiers is also to be avoided: it is suitable for human consumption, but is however to be avoided in aquariums because the pH is unsuitable and the filtration process may reject sodium.
Well or spring water can be used after analysis (may contain unexpected pollutants, nitrate...).
Activated carbon is a complementary filtration that can clarify water, absorb contaminants (e.g. heavy metals), chlorine and treatment residues. However, activated carbon is not recommended in the long term if the aquarium is planted because it also absorbs some of the substances necessary for good plant growth. It does not remove ammonia and nitrite (but there are other resins for this purpose).