Silver dollar, Metynnis argenteus
If the dead fish looked healthy last time you saw it then the first thing to do is to try and discover what happened to it.
First of all check the other fish to make sure that they are all alright, if they all look and are behaving entirely normally. Nine times out of ten this will actually be the case.
Next check the water quality this is vitally important as test results can pin point the cause or rule out lots of thins. Again the water tests will probably reveal that there is nothing unexpected or wrong with the water.
If possible check the dead fish over, this probably won't suit the squeamish but a lot of information can be found by doing so. You are looking for signs of a very recent trauma such as battered fins, missing scales and a general disheveled look about it as this could indicate it had been fighting especially if there is a rather battered looking fish still alive in the tank. If the fish has been dead for more than an hour it is likely that it may be partially eaten in which case nothing can be gained by examining it.
Look for signs of disease, spots, redness, swellings, cloudy eyes, anything out of the ordinary. It is highly likely that this will all reveal nothing.
If the cause can't be determined by going through the above stages and the other fish are all fine then do nothing!
Do not decide to throw in a treatment that you have in stock as a precaution because at best it will do nothing and at worst it could create unnecessary problems for you and the fish.
It is very likely the fish simply died from natural causes which eventually happens to all living things.
Learn how to spot them and how to treat them. Don't treat every little blemish with a battery of remedies and especially don't use any remedy without first getting a proper diagnosis. Using the wrong treatment will:
Recognising the different ailments will enable you to start the proper treatment in the very early stages of the disease which is far more likely to end in a cure rather than a death. It will also mean being able to avoid using an inappropriate treatment.
There is a bewildering array of treatments available to fish keepers. Most are quite effective at treating their target ailments but there is a modern trend to use more natural remedies rather than add chemicals to the tank.
Be very careful about your choice of cure and use a proven effective cure rather than one chosen on a whim, you owe that to the fish which depend on you. Many of the new herbalist, homeopathic remedies are nothing more than snake oil and some are even harmful as such they should have no place in sensible fish keeping.
Be realistic and don't expect a cure all or expect that a few drops of some potion or other will cure everything because it most certainly will not. Tumours, viral infections, severe traumas or organ failures cannot be treated with a few drops of anything natural or chemical.
Start of with as many details as you can, diagnosing fish is very problematic when the fish is in front of you this is magnified several times over when you only have a description to go off.
The information that is important is:
With all this information there is a very good chance that you will receive some good help. If the details are vague and you take a long time to respond to questions as people try to find out more then it is likely that the people trying to help will also lose interest and the fish may die.
This is a scenario that everyone who keeps fish will eventually come across. You take your first look of the day in to your aquarium and find a dead fish. What do you do next and what should you do.