The behavior you describe can be horny or aggressive behavior from the fish- what type of goldfish do you have do you know? Some varieties of goldfish are more prone to bully other particular varieties of goldfish. Its important to ID your goldfish as different goldfish varieties can vary a lot in care and how large they will grow, a link with lots of different varieties of goldfish listed;
http
/www.bristol-aquarists.org.uk/goldfish/goldfish.htm
But it does worry me you only have a small aquarium for these fish- goldfish can grow very large, they're really suited to ponds more than anything else, and when kept in aquariums the aquarium needs to be of a size that will be large enough for the type of goldfish you have and how many- the smallest growing variety of goldfish, the bubble eye, only grows to 5inches long on average, while the largest variety, the common goldfish, can grow to 20inches long (usually smaller though). To keep happy and healthy goldfish, its vital you keep them in a tank that is adequate for them particularly when the goldfish are fully grown. How many gallons/liters is the tank or what is its measurements (length, width and height)
?
Also is the tank filtered? Filtration is another important aspect of goldfish keeping, goldfish are known in the hobby as heavy waste producing fish, they can create quite a strain on the filtration of the tank as they grow as they can produce a lot of waste (goldfish can also eat to up to 2 thirds of of their body weight in food in a day)- if the tank is filtered, what sort of filtration is it and how many gallons/liters does it claim to filter up to?
Cycyling/the nitrogen cycle is another thing you will need to learn about if you are not familiar with this as it is a crucial part of starting up a new tank- although you've had the tank for a few months now, its likely that its still cycling (cycling with fish can take as long as 3-4months). Fish poop and pee ammonia, and this is toxic to all aquatic life. If you put a fish in a tank with no filtration, ammonia will build up and kill the fish eventually (how long it takes to do this depends on many factors), and you obviously don't want this to happen. This is where filtration and beneficial bacteria come in- there is a beneficial type of nitrifying bacteria that converts ammonia into nitrites (toxic, but less harmful than ammonia) into nitrates (not toxic unless in very excessive levels).
This bacteria needs a constant source of ammonia and oxygen and a surface to live on to survive- the ammonia is provided by the fish poop that gets sucked into the filter, while the filters sponge provides the surface for the bacteria to live on, while the flowing water being pulled through the filter provides the bacteria with oxygen. When you start up a new tank though, you don't have any of this beneficial bacteria, so you have to carefully cycle (i.e. establish the bacteria) the tank so the fish can live happily in the tank and avoid having serious bad water quality problems in the tank- even if the water looks clean, it can still have ammonia and nitrites in it as you cannot see these toxins, so its important to test the water with water quality test kits.
Its likely that your other goldfish died of water quality problems- to help ensure your other goldfish don't die, you need to buy some accurate water quality testing kit/s from your local fish store so you can test the water regularly and monitor the tanks cycle and help ensure the ammonia and nitrites levels don't rise enough to seriously harm the fish. If you do not have some already, you will also need dechlorinator/water conditioner- this stuff removes chlorine and heavy metals from tap water and is essential if you are to cycle the tank properly (as the chlorine present in the tap water will kill off your beneficial bacteria). Needless to say, don't wash out your filter in undechlorinated tap water (old water taken from the tank is much better for filter washing), also try and avoid over-cleaning the filter- just clean it enough so that it runs smoothly
. If ammonia or nitrites levels rise above 0 then you should do a small water change on the tank, replacing a portion of the old water with new freshly dechlorinated tap water- never do 100% water changes on the tank as this will completely starve your beneficial bacteria of ammonia and cause the cycle to be prolonged or even make it start from scratch. More info on cycling tanks;
http/www.fishforums.net/content/New-to-t...-tank-syndrome/
In an established tank, ideally ammonia and nitrites should be 0 at all times and nitrates should be between 1 and 40. For the average fish tank, a water change of 25-60% with dechlorinator is advised once a week, coupled with a cleaning of the substrate in the tank and checking of equipment etc (how often you need to clean your filter can depend greatly, but generally speaking you should clean it often enough to keep it running smoothly- i personally clean my goldfish tanks filters once a week in old tank water taken from the tank).
What are you feeding your goldfish at the moment too
? Although goldfish can live off the standard fish flake or pellet, there are so many more things goldfish can eat and which can be very good/healthy for them- an appropriate and varied diet is one of the main keys to good health, energy levels and appearance in fish
.
