pond goldfish disease or parasite or water quality? (ph and ppm tested low, nitrates tested “ok”)?

I have 2 fantail goldfish. One started with brown dime spot on gill
then to the other + scales missing…now they both have it. Then they developed a white coating on their underbelly. I treated for bacterial + fungal one time then the coating went away but still left with brown spots and scales missing. Now the one is darting, flashing, and leaping out of the pond. Started noticing my water hyacinths were dying off. I peeled the dead leaves off and noticed yellow sucking like worms (sticky and slimy) were sucking the life right out of them. Called garden they said to throw them out and that they are a harbor for parasites. having said all that…my q is what parasite or disease causes these symptoms (brown ‘dime-size’ spots on gills peas-size on body, missing scales, darting, flashing, and leaping), how should I treat them? In the pond or quarantine…etc. could it also be ammonia burn? ((sighs)) help