Urgent fish rescue!! Toxic rains?????

We discovered our outdoor fish were very sick/dying tonight..I had thought I'd smelt an odd smell this morning but nothing I could put my finger on. Just like the morning after the heavy thunderstorm 2 days ago..there was a gassy smell unlike any I've smelt before.
As you can see,the pond has had a disaster, within days, I can't understand why, nothing else has changed. The spring water looks and smells OK,but the fish loom like they've been bleached,the water is full of dark oily grit. All feedback or ideas please comment! I've posted a short of the fish we rescued,they seem to be recovering, but we have lost literally dozens!
What are your thoughts? I'd love to hear them.

[@lanterncreekfarm]

Oh no sorry to hear about the fish!! 😮

[@AlgarveAppetite]

Sorry to hear about your fish...good to see that you saved a couple... Sorry I don't have any advice

[@hippiechickoffgridmaine8625]

Oy! I am so sorry to hear of your fish issue! Do you think you can clean out the fish pond? Looks quite large! I heard goldfish. Is that what those fish are?

[@EmbracethechaosPortugal]

@hippiechickoffgridmaine8625 we are draining and refilling, halfway done tonight,but filled with more oxygenated water,tomorrow we will drain again,refill again and hope for the best. I've honestly no clue what kind of fish these are, they were ( or their ancestors were) here in this pond when I arrived here nearly 5 years ago,they belonged to the chap who owned the place before he passed away,I believe they were here for the previous owners too,so 5 years at least before that. Doing their own sweet fishy thing. I hope we can save at least some!