Sick fish. Treatment questions?

[Hertel11]

This is my first spring with a koi pond. We added two new butterfly koi that seemed healthy. Now one has an ulcer/rash (picture posted) but is still eating and not lethargic. The problem is my bigger fish have become lethargic, flashing a lot, and eating less. I don't have a very reliable test kit but one is on the way. There has been a lot of stress to these fish lately though. The pond is about 30' x 35' with a depth of about 3 to 3.5' and much shallower in parts for an average of probably close to 2'. We got 4 inches of rain in less than 24 hours and the liner bubbled up and the pond overflowed. Water was very muddy for about 3 to 4 days and required the addition of a lot of tap water. Just before all this we also added new plants from Lilypons to our bogs and cleaned out muck from bogs. We used API Chlorine and Heavy Metal Neutralizer as directed while adding new water. Now, a week later the water is back to crystal clear but the big fish seem unhappy with no noticeable sores. Big fish are hanging out on bottom and in corners a lot, not eating much, and flashing frequently. My question is; should I treat the whole pond with Melafix? My best guess is the pond is about 20,000 gallons. Is it possible the fish will recover on their own? What should i do? Please help. Right now I do not have a seperate tank to quarantine fish in but I suppose I could get one if needed. My plan is to treat the whole pond with Melafix for 7 days as directed unless others have better suggestions. Thank you.

 

[Hertel11]

Everything seems to be at normal levels...

 

[WaterGardener]

I would do nothing until you get good testing, the liquid drop tests. Honestly, I can't tell anything from that test strip. I suspect this is a water quality issue with possibly parasites.

For parasites I like to use Fluke-M. It doesn't hurt to have some on hand.