Change water early or double dose Prime

[NofishB4]

I noticed a couple of my fish had some stringy poop and was watching them. I’m located in Canada so meds are not readily available to me. I have 1 box of API general cure (10packs). Yesterday my female peacock gudgeon started flashing periodically and I did a 50% water change. Today she is still flashing and also has stringy poop. So I decide to dose the whole display tank. Bear in mind that none of my fish have been medicated (to my knowledge) before...although they went through quarantine at my LFS and at my house. 
 

So I notice one of my honey gouramis swimming erratically and belatedly test my tank water (which I changed yesterday). My ammonia is 0.5!  Not sure where it is coming from (tank is cycled and running for a year). The only thing I can think of is that my male peacock gudgeon might have passed away (he disappears all the time to nest and I’ve never found where) and is causing the spike. I’ve moved all my decor and can’t find him and everyone else is accounted for. 
 

So my question is should I do a 50% water change (tap water has 0 ammonia...I checked) and not give the full dose of meds (tank is 38G) or should I wait the 48hrs, double dosing with prime and then do a 75% water change and continue the med schedule?

So far I have not seen my female flash again (lights are off) and my honey is resting on his betta leaf. My other fish seem unaffected. 

Edited December 29, 2020 by NofishB4
Typo

[Bsharrow]

 

I think the best approach might be to deal with the issues one at a time.

If I were you I'd take care of your ammonia problem first and then continue dosing general cure. You'll probably notice your fish get a bit better once that ammonia is gone.

I'm not sure what will happen with general cure of you double dose prime. Someone else will probably be able to help you with that.

I live in Canada too so I know how hard it is to get general cure so you don't want to waste it.