Hi Elz
That's very good advice from Silverfish. Given the water change history I'm going to suspect high nitrate is to blame for his lethargy.
Undertaking an emergency 50% water change today and then 4-5 days of daily 25% water changes I'm sure will help the fish feel far perkier until the test kit arrives

It can certainly do no harm at all.
Don't wait for the kit before you start water changes, we can advise as and when when you get it.
I have 2 fancies also in a 180 and undertake roughly a 40% change each week. My tap water nitrate level is nil, but in the tank this rises to around 20ppm in a week.
Some goldy keepers prefer to do twice weekly changes as usual maintenance, to keep nitrate down.