Just a hint for the future. There is no need to change your filter wool until it becomes so glogged it won't work anymore. It may look gross but it is actually a healthy colony of biobugs that breakdown fish waste into non-toxic form.
I'm not sure how much water you change every 2 weeks but 100% water changes are not recommended except in cases of emergency. Once again, if you change 100% of the water you are removing the good biobugs. A safer routine is 30-50% weekly water changes using a siphon to vacuum the muck out of the gravel.
Slime disease has many causes, including parastical and bacterial infections. Your fish may even have nitrIte poisoning but we cannot tell without a water test reading. Sadly, once they start lilting on their side the prognosis is not good.
I could recommend daily water changes of 30% in case there are high levels of ammonia/nitrIte in your water. This could occur as a result of replacing filter material and loosing all the good bacteria. Another tip is salting the water to .3%. This needs to be done gradually, over 3 days. Each day dissolve 1 small teaspoon of salt per gallon and add to your tank, this equals a .1% solution. Repeat for another 2 days and you have a .3% salt solution.
Salt will help goldfish handle nitrItes and will also kill most parasites. Aquarium salt, rock salt, sea salt and kosher salt are all ok to use. Do not buy conditioning salts as these contain different minerals and are used for a different purpose.
It is also recommended that you continue 30% daily water changes while the salt is in the tank so you need to add extra salt to compensate for the removed water.
I'm sorry but that's all I can think of, it may help your fish if it isn't too far gone.