Where I live, I can't access antibiotics or many effective meds for fish. This makes me proactive, and I haven't had to deal with popeye for many years, since I switched up to weekly partial changes.
I would read a few sites on using salt, as soon as I could (because infections don't take time off). I'm not sold on salt for an internal infection, but since water flows through the sinus, maybe. Epsom salt baths can sometimes cause the fish to let the fluid buildup out. Then, if you're where I am, you hope clean water will give the fish a chance to heal on his own.
Most of the time, salt works as a skin irritant, which makes the fish add to its slime coat, and makes it hard for skin parasites to hang on. Fish slime is a powerful immune response. But inside the head? I'm not sure about that.
Test kits will give you a look at the nitrogen cycle, but it won't tell you about solid wastes not yet broken down, or the many other chemical processes when living things are in water. They aim at the most fatal one - ammonia, but there is more to it all than that.