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Generally speaking, fish live with a low-level infection of various bacteria and parasites all the time, and the fishes' immune system suppresses any problems of infection that arise. If a fish's immune system is stressed, then infections and parasites are given an opportunity to develop and start to cause health problems, which further stress the fish, and so things worsen.
The commonest cause of stress is poor environment, especially poor water quality. In some cases, infections and/or parasites are introduced into aquaria with new fish or plants that have not been quarantined properly. Generally, over 90% of disease problems are due to poor water quality, with less than 10% being due to introduced diseases/parasites or other causes; poor water quality means insufficient dissolved oxygen due to overstocking and/or too many organic wastes.
Understanding the above is the key to successful fishkeeping. Here, then, is some 'off-the-shelf' general advice on goldfish keeping; we hope it helps.
First, if you haven't got one already, quickly go out and buy a basic book on goldfish keeping - such as the Interpet Guide to Fancy Goldfishes by Dr Chris Andrews, about £6 - and get reading. (Chris Andrews ran the London Zoo aquarium and then the US National Aquarium in Baltimore). (The cost of the book will save more than the cost of ignorance of the rules).
Frequently, people's goldfish tanks just go badly wrong within the first few months of setup. The water may look crystal clear and there may be no apparent reason for trouble, but the goldfish decline into worsening ill health with fungus, finrot, shimmying and listlessness. Replacing the fish doesn't work, because the result is the same. Rescue is to be found in understanding some very important relationships that bear upon fish health and stress.
These are the four fundamental things to get right:
Stocking level - what are the SURFACE dimensions (length x width) of your tank? As a guide, in a coldwater aquarium, you should allow:
For example, in a 60 x 30 cm (24 x 12 inch) tank you can keep a total of 30 cm (12 inches) of combined fish body length, such as three 10 cm-long fishes (three fish each 4 inches long), or two 15 cm-long fishes (two fish each 6 inches long).
There is no way round this! If you overstock, you will always be up against problems; remember, too, that small goldfish grow, so you need some spare capacity for growth of your existing stock. Do your sums, and, if you are overstocked, find temporary accommodation for your surplus fish until you either get a second or a larger tank.
See our Aquarium Calculator to help you determine safe stocking levels and for further advice on this subject.
Filtration - goldfish produce a lot of waste and as a general rule undergravel filtration cannot cope with it. There is no way round this! If you have just an undergravel filter, leave it in place and running but buy an internal power filter or an exterior canister filter as well (Interpet, Tetra, Eheim, Fluval, etc - tell the retailer your tank dimensions and he/she should offer you the correct model). You could also buy a filter bacteria culture (sold in plastic bottles by Hagen, etc) to start the new filter off. If you have an undergravel filter, you must also have an air pump to be running it - good, as this provides aeration.
The commonest aquarium problem is inadequate biological filtration on account of:
Not having a filter at all means you must rely entirely upon frequent, large water changes (see below) for the health of your fish.
Feeding - if you have a water quality problem, stop feeding until the problem is over, to reduce the level of wastes in the water. Then resume feeding very lightly: two or three small feeds every day are better than one larger feed as there is less uneaten food and the fish then scour the bottom for leftovers; any food not eaten up in two or three minutes is too much. (Goldfish can go for up to two weeks without food - for example, when you go on holiday, it can be safer to leave fish unfed rather than risk a neighbour overfeeding them and polluting the tank).
Feed a good flake food such as Aquarian or Tetra goldfish flake daily. Supplement flake with live or frozen foods (frozen is just as good, and easier!) no more than once per day for four or five days per week. Overfeeding is the commonest mistake in fishkeeping, according to numerous surveys. In fact, one or two days per week without feeding anything at all can help towards better water quality.
For further information, see Feeding under our General FAQ.
Water changes - if you have a water quality problem, change 50% of the tank water every day until the level of nitrate (see below) drops to about 25 ml per litre (use a test kit purchased from an aquarium shop), and thereafter change 25% per week every week, for as long as you remain a goldfish keeper. Use a water conditioner (Tetra, Waterlife, etc) to treat tap water, to remove the chlorine/chloramine (put in by the water authority to disinfect the water, to make it safe for human consumption) and any dissolved heavy metals.
Fish excrete ammonia (their equivalent of urine) via the gills into the water, which is invisible and toxic - so clear water does not necessarily mean safe water. Filter bacteria then convert ammonia to nitrite then nitrate (this being known as the nitrogen cycle), and the fishkeeper must remove the nitrate - there is no way round this, too!
There are three ways of removing nitrate:
Rely on water changes, with good plant growth as an extra if you can achieve it; technical means include chemical resins (placed, for example, in a nylon bag in your filter) to absorb nitrate, and de-nitrating filters - both are expensive and can go wrong.
Always clean the internal/canister filter media (foam pads, filter wool, etc) in a bucket with water taken from the tank, as water straight from the tap will kill the filter bacteria (because of chlorine). The goldfish book (that you should buy) will have diagrams and charts explaining all this if you are not familiar with it.
Look around for a knowledgeable retailer and for a goldfish club, if you can. It's worth subscribing to Practical Fishkeeping Magazine in UK (or overseas equivalent!) to keep up to date with advances in fishkeeping techniques.