Welcome australiancomet to the forum, lots of nice people here, all happy to help. So you're in Down Under too?
But let's get started to solve your problems.
Nabi and Andrea covered the most important already.
1. The tank you've got is faaaaaaaaaar to small for 4 comets. You need min 10 gal per goldfish. They grow fast, they can reach up to 12 inch or a bit more. They also are very "messy" fish. This means they eat and are champs in pooping too, lol
This is why you need a bigger tank, a 3ft would be perfect. Enough water and space to swim, weekly maintenance with 50 % waterchanges ok.
2. You need other equipment towards your tank: a very good filter whcih turns at least 2-5 times the volume of the tank per hour, others recommend even 10-20 times. This will be full blast
. Then you will need an airstone to oxygenate the water and of course a pump.
3. Live plants. Not only that goldiel like to nibble on them like a salad snack, but they also use up nitrogens ( nitrate ) and produce oxygen during the day ( see Photosynthesis )
Now that your fishies are bottom sitting is a sign of distress which will lead to illness. Most times poor water quality is the reason for this. And if it is not acted on this matter the fish will be poisoned by their own waste and poorly die
Did the staff in the petshop inform you about the Nitrogen Cycle ? And that the tank must cycle before adding fish? And to add one fish, maximum 2 at the time? 4 fish at once are definitely too much bioload for the filter to manage.
At this stage your fish will poison themselves, especially in only 17l. ( which is even too less for 1 goldfish )
What you need to do to help your fish to survive and get better is the following:
Get yourself a bigger tank ASAP with filter and air supply ( look up on ebay, trading post, local forums... we do have quite a few here in Aussie... if you are interested let me know, please )
In the meantime 50% waterchanges EVERY single day. But please don't clean the filter down, let the beneficial bacteria settle.
Add waterconditioner like "Stability" from Seachem according to instructions. Add waterconditioner to remove chlorine and chloramine from tap water which are highly toxic to fish. Keep pH right, with this one it is most important that it does not fluctuate like a yoyo, at the same time a pH of about 7.0-7.2 keeps Ammonia less toxic.
Get the API freshwater test kit and keep track of Ammonia, Nitrite, Nitrate, pH.
Now, if you live in QLD, maybe @ the Sunshine Coast I can recommend a local supplier for the best waterconditioner going or even join our local "club" of fish crazy peeps.
DO you know about the Nitrogen Cycle? If not, let me know, I'll explain it to you and how everything connects together. Please don't hesitate to ask.