No matter how good you are at looking after your aquarium, there's always the possibility that something will go wrong and you'll find yourself out of your depth. Many people look for advice on the internet for the first time after finding themselves in this sort of trouble. Preventative measures are always best, but when it's too late for that, what can you do?
If your water has turned dark, if it's started to smell bad, or if your fish have suffered from a series of disease outbreaks, there are several basic things you need to check:-
The pH of the water. If this is too high or too low for your fish, a special buffer solution, which you can order through your local fish shop, can provide an emergency solution. If you don't know what pH your fish prefer you can look it up online.
The salinity of the water. Different saltwater fish prefer different salinity levels. Often salinity increases over time, even in freshwater aquariums. If salinity is too high, change a fifth of the water and re-test. Scrape away any salty deposits at the waterline and remove them from the tank.
The ammonia levels in the water. If these are dangerously high, change one fifth of the water, treating any new water with a dechlorinator and making sure pH and salinity levels remain steady, then repeat the process on each of the next three days.
Temperature. Make sure your heater is working properly and that you're keeping the water at the right temperature for the specific needs of the fish in your tank.
Feeding. Unless your fish look seriously underweight (with ribs visible against the skin), cut down the amount you're feeding them and, unless they're exclusively carnivorous, try to give them more vegetable matter in their diet.
Overcrowding. If there are simply too many fish in your tank, you may need to remove some of them so that the others can live safely. Fish shops will often accept fish from you in situations like this. Never release fish from your aquarium into the wild.
If your aquarium ecosystem is seriously damaged and your fish are unable to remain there safely whilst you repair it, set up an emergency tank and fill it with new, treated water. Adjust the pH with a buffer solution and make sure the temperature is close to that of your original tank. If possible, add some new plants and rocks (purchased from a fish shop) for your fish to hide behind, as this will reduce stress. Do not transfer any plants, rocks or ornaments from your old tank - it's usually best to abandon your plants and anything else should be boiled to kill of disease organisms. Live rock will have to be replaced when you're ready to start up again.
Water in an emergency tank needs to be changed frequently in lieu of a properly functioning ecosystem. It's especially important not to overfeed in this environment.
When fish are suffering from several diseases at once or from stress-related minor infections, the best thing to do is usually to help their immune systems to fight back the natural way. Slightly increasing the salinity of the emergency tank will often help. There are several treatments available which help to restore damaged mucus membranes and these can be invaluable in giving your fish a fighting chance. Steroid-type treatments like Melafix can be used in almost any short-term situation and improve healing. If your fish are not eating, try a liquid fry food to provide them with easy nutrition. Be attentive but don't fuss over them - they'll need peace and freedom from stress in order to get better.
Often disasters like this can feel like the end of the world, but if you respond in the right way, it is possible to recover. Then it's time to start doing your reading and making sure you're sufficiently well informed to prevent it ever happening again.