bunny, the first thing I noticed in your post was you stated you do a 50% wc every two weeks. This isn't nearly enough, especially with an overstocked tank. With your tank overstocked and ill fish, you need to be changing 50% of the water at least 3 times a week. If you read around the forum here you will see that fantails (double-tailed goldies) require a minimum of 10 gallons each and comets (single tailed) have a minimum requirement of 20 gallons of water. So if you have 2 fantails and 2 comets that's 60 gallons
minimum. You also stated that you did a wc yesterday. Was this also 50%?
I also noticed that you posted an ammonia reading of 0, but no other readings for your nitrites, nitrates, etc. These readings are also crucial as it lets you know if your tank has had a cycle crash for some reason. If you don't have a tester kit there, you can take a sample of your water to a lfs and ask them to test it for you. Just please be sure they give you the exact readings of each category.
Adding any medications to an overstocked tank with poor water quality isn't going to work. There are different medications for different fish problems, just like with humans and animals, and using meds in toxic water can actually make the problem worse.
One more thing. The Aquatech 20-40 is too small. Goldfish are nasty little critters and put out much more waste than tropicals. Therefore, they need lots and lots of filtration. I have a 30 gallon tank with 2 filters on it that pump 400 per hour (I think thats the amount it said).
Wow, this looks like a lot, huh? It really isn't, though, as the main thing that you need to do is an immediate large ph and temp matched water change. But we really need the rest of your water params or its just like shooting in the dark.
Oh gosh, I just saw where you posted that the conditioner you are using is aquarium salt. Are you using a dechlorinator? I assume you are since your tank has been running for 5 years.