When you set up a new tank the filter media / sponge does not contain any of the bacteria needed to break down fish poo and wee. The poo and wee can build up quickly, especially from goldfish who are dirty buggers. It breaks down into ammonia, which in a mature tank is 'eaten' by the filter bacteria and changed into less harmful stuff. Being new your filter has none of the bacteria it needs so your fish have more than likely died of white spot due to high ammonia levels in water. If I remember rightly, wait for others to reply thou, you can have one goldfish per 10 gallons of water as they get relatively big and produce a lot of waste. That means your tank can have about 5 or 6 fish eventually IMO. BUT you need to add them slowly.
My advice.
* Treat your tank with white spot med using instructions.
* Get a liquid master water test kit to find out your water stats.
* Leave one or two in tank for around 2 weeks. Do small water changes 10 - 15% 2/3 times a week to keep harmful stuff down. Cant be completely sure how much or how often without you water stats (Ammonia, nitrite, nitrate) which you can only give us if you get a liquid master water test kit. After 2 weeks add another fish and repeat routine for another fortnight. Then add another, repeat etc. until you are fully stocked with you 5/6 goldfish. Only by doing this will the bacteria you need build up in your filter. Gradual addition of fish over many weeks means that the existing bacteria can cope with the addition of the extra poo / wee produced by adding your extra fish. Chucking loads of fish in straight away will overload your bacteria who wont be able to cope with breaking down all the waste.
* Once set up and fully stocked which will take many weeks. Change 25% water weekly with dechlorinated water of same temp. using a gravel vac.
* Squeeze out filter wool in old tank water weekly.
* Squeeze out filter sponge monthly in old tank water or if it gets mega clogged up.
* Be careful not to over feed. Especially goldfish who are extremely greedy. Feed a varied diet: flakes, cooked pea, frozen blood worm, frozen daphnia once a day. Only enough that they can eat in 2 mins.
* Next time fishless cycle using the add & wait method. Read this:
http
/www.fishforums.net/index.php?showto...3861&hl=methods
Good luck
