Hopefully this will help people properly care for their goldfish and give them the best life possible.
Goldfish are one of the most widely abused pets owned, due to the common misconception that they are easy to keep, grow to the size of the tank/bowl and because they are cheap to replace if they die. Infact, goldfish are a large fish, capable of reaching 20" given proper care and space,and have been known to live up to 40years, although 15-20 is much more likely. They have great personalities, and a good memory. Some have even been taught to do simple tricks for food!
Aquariums for goldfish need to be large to allow the fish to grow to the full potential its genetics will allow. Tank size is very important,it must be carefully thought out as it greatly influences the potential of the fish to reach its adult size. A stunted fish will die much earlier than a fish kept in a large tank allowing them room to grow.
Goldfish can become prone to several health issues as a result of the poor water conditions in a small tank/bowl. Unfortunately, they produce a large amount of waste especially when their metabolic rate is increased due to elevated temperature. As a result, the water quality deteriorates rather rapidly especially when the tank is less than ideal in size, even in a large tank, water changes and gravel vacs will be your best friend in keeping your fish fit and healthy.
Considering goldfish are not designed to stay small (which is why fishbowls are considered cruel) and at full grown can be in the 8-10 up to 18-20 inches range depending on type, a general guideline for fancy fat body types should be at least 15-20 gallons for the 1st fish plus an extra 10 gallons per fish in the tank after that, and double that for common/comet/shubunkin types.
This does not mean that the spare 15 gallon tank you have can be used for a goldfish, they are very social creatures and you should not keep goldfish in isolation as they are more unlikely to thrive if deprived of company. Minimum tank sizes for goldfish IMO would be 25-30 gallons for fancies, and 50 gallons for commons as you will need at least a pair. Bigger is always better, and just because a fish will fit in your tank, doesnt mean it will be able to swim properly, and have the space to move and use its muscles. Rectangular tanks are always better as they provide the largest air to water surface for oxygenation. Goldfish have high oxygen requirements, yet another reason bowls are completely unsuitable. All in all, the bigger the tank you can get, the happier, healthier and longer your fish will live.
Here is a list of some types of goldfish.
Fancy goldfish-can be slower than commons-
Black Moor/Telescope
Fantail
Oranda
Pompoms
Very fancy goldfish-can be much slower swimmers-
Lionhead
Ranchu
Eggfish
Celestial
Bubble Eyes
Common or pond type goldfish-fast strong swimmers-
Comet
Common Goldfish
Shubunkin
It is advised not to mix commons with fancies, and especially not very fancy goldfish. 2 main reasons are-
Competition for food, the commons will in general be faster stronger swimmers than the fancies, and definately faster and stronger than the very fancy fish. The fish with eye growth such as bubble eyes, celestials, moors wont be able to see the food aswell as the commons either,aswell as some orandas and lionheads with large wen growth.
Spawning, male common shaped goldfish will easily out swim a female fancy goldfish, and will push her to exhaustion if they are together when she starts to spawn. The males can be very hard on the females, shoving them around and into plants etc, having a large strong male goldfish pushing a smaller female goldfish around is never going to end well.
Males can be sexed from females by means of the breeding dots on their gill covers (not always visible out of spawning mode, although my males have always shown them at all times) and also by venting. Venting is looking at the fishes vent (where they poop from and also where females expel eggs from) the males should be flat and the females can have a tiny little bump, its becoming quite easy for me now to sex even very small goldfish via venting.
Fancy goldfish are far more suited to aquariums than common goldfish, mainly due to their adult size. Ponds, either indoor or outside, should be considered for commons/comets/shubunkins.
Stunting is one of the most talked about subjects when it comes to keeping fish. It remains a debate with different theories as to what causes stunting in fish growth. There are different things that influence the growth rate of a fish.
Genetics obviously will play some part in this, if you have a small mother and father, the off-spring will not be as large at adult size as that of larger parents. Any tank should allow for the maximum adult size of the particular type of goldfish, and not be taken as you can go smaller as the parent fish were smaller, it could just be that the adults spawned before becoming fully grown.
Stress is another factor that is sometimes missed when it comes to stunting. When a fish is stressed, it releases hormones in response to the stress, which reduce the growth hormones. Once the stress has been removed, the fish may have a growth spurt due to the fish releasing more growth hormones again. A fish may stunt through stress while in a smaller tank, but then when placed into a larger tank could make up some growth, although will never be as large as it could have been had the stunting not taken place,and no doubt the life span will also be reduced.
Overcrowding can also cause stress.Many people tend to buy a lot of small fish and forget that their fish are still sub-adult. The lack of space can prevent the fish from developing normally as they should do as all the fish will release the stunting hormones, so the more fish there are in the tank, the stronger the hormones, doing very regular partial water changes will help dilute the hormones but in the long run stocking will need to be reduced.
Water changes are a must, in all tanks, to dilute the nitrates that build up and also the hormones released by the fish. Gravel vacs are a very good idea too as the debris (food/poop) that drops down into the gravel will break down and further pollute the water. Goldfish poop a lot, so a gravel vac i would say is pretty much compulsary!
The nitrogen cycle is explained already in different threads so i wont go into that, but its a very important process in every aquarium.
Test kits are a fantastic product, fairly cheap to buy (dont buy test strips as they can be inaccurate) API do a good liquid drop test kit which is easy to come by and is a good price.
Most people tend to forget the nitrate will remain in the water elevating dangerously if not for the regular partial water changes. I carry out around 60% every weekend in my goldfish tank, with a gravel vac. Nitrate should not be allowed to climb higher than 40ppm. Try to set-up a scedule for your water changes so your levels are always <40ppm before a water change, preferably, nitrates should be kept at less than 20ppm, and in heavily planted tanks the nitrates can be at zero even in a cycled fully stocked tank. Heavily planted tanks very rarely have goldfish in them though, as the goldies will simply snack away until theres not much left,good for the fish, not so good to look at. Water changes will still be needed but not so regularly if your tank is heavily planted. In high nitrate tanks, fish can become more susceptable to health issues, and can also become stunted. Even in a planted tank, you will need to water change to remove some of the fish poop etc from the gravel and keep your water in top condition.
Filtration is extremely important. Goldfish have a reputation for being very messy compared to tropical fish but this is not always true,plecos and large cichlids for example are also very messy.
Aim for a turnover rate of around 4-5 times the water volume of your aquarium. My aquarium has a turnover rate of around 7x the tanks volume per hour by means of 4 separate filters.There is no such thing as too much filtration, unless the out-lets are too powerful and the goldfish are swept around and get stressed in the process, this can be avoided by adding spray bars to the out-lets to disperse the flow more gently. Keeping goldfish is certainly not a cheap hobby. If you cannot afford the proper set-up, maybe try smaller cold water species such as white cloud mountain minnows for example, or maybe even a smaller tropical tank with rosy barbs as these can look quite similar to goldfish but remain much smaller.
When it comes to cleaning the filter media,what you should do is use your old tank water or clean dechlorinated tap water in a clean bucket to rinse out the filter media. It does not have to be spotlessly cleaned as most of the dirt in it is still beneficial for the tank and will hold bacteria, the majority of your beneficial bacteria will live in your filter media. You should not replace the old filter media with new ones unless the old media is literally falling apart, and even so, try to retain a small portion of the old media to put in alongside the new so that the bacteria can more quickly colonise the new surfaces.
Diet, very important, especially for fancy goldfish. Fancy goldfish are prone to bouyancy problems due to their internal organs being cramped up in their deformed bodies. Floating foods should be avoided at all costs. Quality sinking pellets are by far the best option of shop bought foods. I use saki hikari food for mine and im happy with the results. Flake foods lose their goodness very quickly on contact with the water, so even if you put the flakes under water to stop them from floating, they have very little nutritional value compared to a pellet food. Another option you can look into is gel foods. Some people use these for their fish as they can control what goes into them and they have zero gas content in fish which do have floating issues.
Goldfish also need a lot of greens in their diet, either by means of live plants, or by adding foods to the tank a couple of times a week. My goldfish adore cooked shelled peas, and cooked broccoli. They get peas at least twice a week as they are thought to push out any trapped gases in the intestinal tract which can create floaty problems, i know when i feed them peas, the bright green poops the next day do occasionally have gas bubbles trapped in them.
Variety in their diet will always do fish good. Mine get saki hikari pellets as their staple diet, then they have shelled cooked peas, broccoli, sweetcorn (squeeze out the squishy centers into the tank),courgette which they pick at,live or frozen daphnia/brineshrimp/bloodworm once or twice a week too. They also have live plants in the tank which they enjoy snacking on too.
With fancy goldfish especially, tank decor is something to seriously put a lot of thought into. Any ornaments which they can swim into and get trapped, should be removed. Any ornaments or plants which are sharp can easily injure these fish, and fish with eye bubbles can get caught and the eye bubble then pop, usually the fish will heal up but wont look as nice, and can become infected with the open wound, fish with protruding eyes such as moors can be blinded by rough edges too. Silk plants are best if you use fake plants, and also driftwood needs to be rounded off, so the sharp edges are all gone.
Tankmates for goldfish, well theres other goldfish, dojo loaches, and bristlenose plecos, i always worried their sharp spines on their faces could harm my goldfish but have so far had no issues and they dont bother the goldfish at all. Apple snails are good with goldfish though may get nipped at, and nerite snails with smooth shells have also been kept successfully with goldfish. You can also house white cloud mountain minnows with goldfish, but 1 day chances are they will become a goldfish snack.
Do not add common plecos to a goldfish tank, they have been known to latch onto the side of a slow moving goldfish and eat away at its slime coat. I have seen this myself and i will never add one to a goldfish tank for this reason. It may be ok if you have a tank of just common goldfish and they can all move quickly. Personally i wouldnt try it.
I hope this helps anyone wanting to know a bit more about their fish, and ive made any mistakes or missed anything, feel free to point them out.
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Subject: Goldfish Care 
Sat Feb 04, 2012 11:18 pm