It's good that you are replacing 30% of your water (PWC) weekly but are you vacuuming your gravel really good with each PWC? Are you doing filter maintenance by cleaning the heavy detritus off of your filter media on a regular basis? (I do mine weekly or at least every two weeks). If you are not doing this, the decaying detritus in your gravel and filter media would cause you to have excess nitrates. It could also be coming from your tap/source water. Excess detritus in your gravel can also become a breeding ground for bad bacteria and we know how goldfish like to forage in the gravel. It could be sucking up lots of bad stuff while looking for food.
It's good to have a baseline of test results on your tap/source water and check it a few times a year. To do this, run your cold water for a minute to flush out any stagnant water that was in your home pipes. Then fill a gallon jug with water. Test it for ammonia, nitrite, nitrate, pH, KH and GH and record your results or post them here. Then add your dechlor product. Test for ammonia again. Post the test results. Then wait 48 hours and test for everything again and post your results. Quite often, you will see changes in the pH, KH and GH due to the buffers in the water losing their buffering capacity. This gives you a better idea of what will happen to the water once it's in your tank where even more biological processes take place.
Some people also find that they are getting high nitrates right out of their tap.
If you get any ammonia, it is probably from the chloramine that is used by many utility companies to disinfect drinking water. I have a risidual of 0.5ppm of ammonia in my tap water. When I do a PWC, that small amount is instantly consumed by the nitrifying bacteria in my filtration.
Now, on to your tank stocking.
You have a mix of goldfish and tropical fish which is not always a good thing. They have different needs for their water parameters. Long-bodied Goldfish like sub-tropical temperatures below 70F. Fancy Goldfish like sub-tropical temps in the low to mid 70'sF where tropicals need the high 70's to low 80's. If I remember correctly, yours is a long-bodied goldfish.
Your tank really isn't big enough for long bodied goldfish. They grow to 12"+ and are big-time swimmers so they need lots of swimming area. I know yours is still relatively small but keeping it in an undersized tank will cause it to get stunted which can also lead to the health problems you are seeing.
You need to find out what kind of catfish you have. There are thousands of different species. Do you remember anything about the name of them when you bought them? Did you buy them as algae eating catfish? Can you take a picture of them and post the pic here or provide a link to your photo so we can help figure out what you have.
http
/fish.mongabay.com/species/Carassius_auratus.html - Goldfish Profile
http/fish.mongabay.com/species/Gymnocorymbus_ternetzi.html - Black Widow Tetra Profile
Water recommendations for the known species that you have:
Goldfish - pH 6-8 (7.0), 4-25 dH (10), 46-68°F (8-20°C)
BW Tetras - pH 5.8-8.2 (6.8 ), 4-28 dH (8 ), 72-82°F (22-28°C)
While the pH and dH (another way to read hardness) are similar enough, the temperature recommendations are drastically different. What temp are you keeping your tank at?
Keeping a fish too far outside of it's preferred water parameters will cause stress to the fish which then leads to a weakened immune system and potential health problems.
It would be best to treat the goldfish in a separate tank but if you don't have that option, then you should pick up some anti-bacterial food and put the goldfish on that for 7-10 days to cure the bacterial issues you are having. Since you mentioned the infected eye also, I would suspect the swim bladder issue is being caused by internal bacterial problems as well. I'm not familiar with what medicated foods may be available to you in AUS so go to your LFS or pet store and write down the names and ingredients of the available foods and post them here. A common one that can usually be found is Jungle's AntiBacterial food. It's certainly not the best but it would help a little. There are much better one's available online but I'm not sure if they are available down under.
I bet you didn't realize there was so much to know just to keep a few fish.
