Hello...
I have a large (8 or 9 inches long) solitary shubunkin. Had it for 3.5 years. I have softened well water...I test it each time I do a water change, and it is always exactly perfect (all numbers are dead-center middle of the "safe" requirements on the stest strip bottle). My tank is 20 gallons. I have a Whisper filter that specifies for 20-40 gallon tanks, and it uses two filters in it..one is just coarse foam, the other has activated charcoal in it, changed monthly. I feed pond fish pellets and live insects like spiders & flies (my daughter loves to catch these yucky things, but the fish loves them even more) however, its only maybe 2-3 times a month that we get live insects.
So....i did a filter cartridge change (the charcoal one) and after that, for the last 4 days, my shubunkin has been laying on the bottom of his tank. He does get active sometimes, but only short bursts. he is not wanting to eat his pellets. I thought the charcoal might have something to do with it, so I got a temporary 10 gal tank ready & put the fish there overnight. I cleaned his 20 gal tank out completely, and re-set it up, minus his bubble rock because it needs new tubing. He is back in the 20 gal, but is still laying on the bottom, breathing. He has poop protruding, thick, same color as his food. I am out of ideas. ...I am attempting to post a couple pictures of my poor shubunkin... Thank you

Oh my, he doesn't look very happy.

Bottom sitting is more often than not a sign of stress or illness.
I completely agree with Scrivens, a much bigger tank will do the fish some good, or even better moving it to a pond once it's recovered That fish is just massive and I'm sure it'd like more swimming room for exercise!

Keep an eye on the water parameters and follow Scrivens advice on the water changes
Best of luck, keep us posted.

Don't give him to a pet store, you never know who they'll sell the shubunkin too  

Instead, why not look up local neighbors or people with ponds? Or perhaps a local park that has a lake which can accept fish? Do not release him into the wild (it's illegal), check with local authorities before you go putting it in any park lakes or ponds!

There may be a fish rescue service near you? Looking on google I found this; (Link is not visible to guests. Please register to view.) So perhaps drop them a visit and see their facitilities?  

Cheri one of our members lives in the USA and knows a lot of fish people, so I'll drop her a line and see if she can help  

How have the water changes been going? Any change?

Heya, heard back from Cheri and this is what she's suggested:

"Ohio is quite a ways from me. I'd suggest she tries local Japanese gardens, parks or zoos that have large ponds. I've also been able to meet people through the pond and garden stores. As a last resort someone on Craigslist will take him- just suggest she interviews them, you know, depth, filtration etc- the usual. Sorry I couldn't be of more help"

Fantailer the fish is already 8 inches, it is definitely big enough for a pond