I am going internal infection with the head down attitude.
I am seeing an extremely bloated fish with the protruding vent and lateral line as the main points. Whether it is from eggs or internal organ failure I cannot say. With this age of this fish any attempt to spawn (after five years of no spawning) would probably cause some damage internally.
Well, my ignorance is responsible for that. I didn't know that females continue to produce eggs even without a male prompting her. After the last spawn and my hubby freaking out over the smell, we took many of the adults and put them in the outdoor pond (probably all the males). Spawning may have been 4 years instead of 5 but the bottom line is maybe the one or two new guys are just getting around to maturing enough to try out the equipment so to speak? But not big enough to take her to the dance?
All the eggs are never fully absorbed, especially in an indoor pond and probably would require a fast of several weeks to even start this. With the floating foam I am sure you have a high concentration of DOC's and probably NitrAtes.
Yes, we have Well Water and it comes in with NitrAtes, so it's a never-ending battle.
Also we don't have a bottom drain. Which is why we have 2 filters for a 1700 gallon pond and do a 35% water exchange each week. I'll attach a photo of the water tank we use for fresh water/water exchanges. Blue fiberglass. Got it from Aquatic Eco Systems many years ago. Holds 550-600 gal. depending on how high you fill it.
However if you are doing a weekly water change of 600 gallons (35%) this would not make much sense.
Part of the reason for that now is the one filter (Ultima 4000) NOT being backwashed for the last 4 weeks because we just pressure washed the insides, put in all new media, a new lateral column, etc.) so while trying to seed the media, I imagine there's quite a bit of muck building up in there adding to the NitrAtes & foam too. I also assumed (maybe terribly wrong) that the .3% salt and the treatments of Ultimate water conditioner is further contributing to foam.
I know the Phoam Fractionator goes to overflowing even when I have it tapped down to the lowest setting. Just a possibility?
The other strange thing is if your "new" fish are five years old from a spawn you had. Why are they only 7-12"?
My Bad. Middle of the night, trying to write the thread & picture them in my head, and I figured I'd give a general broad range. We do have one little butterfly runt from the spawn (not counting tails in measuring) he's actually about 10" long now and the others are between 14 - 19" It was never an important factor to me, you know? We were heavily overstocked before the spawning and thinned out a lot of adults and 99.9% of the eggs after that.
Is that still small? I wouldn't know.
They should be much larger than this. Something is amiss and that something is probably why your fish is not well.
I'm sure there has to be a reason, but with all due respect I hope you're wrong about the correlation between the size of my fish and why this particular fish is sick.
Because if it is in fact the same cause, each of the rest of them are awaiting this very fate in turn. I'm sure you understand my reasoning there, right?
That is about all I can say. Not much help but I am not sure there is much help for this fish at this age. Hopefully someone else sees something I do not or an actual pond visit might reveal something you might not have mentioned.
Good Luck.