Though I’ve read that their are great aquarists on here it would be prudent, if you don’t receive advice soon to call a vet that cares for fish! From what little I’ve read and remember Kimpy could be suffering from ammonia burns (even though it tested at 0) or stress from overcrowding and bullying. Have you tried 50% water changes daily or even a few times a day along with treatments? If it is a fungus you need to toss the gravel, boil all the toys, replace the hoses, purge & sterlize the pump (Hot water & bleach followed by ten gallons of clean water through pump.) The filter media needs to be replaced, the filter housing needs to go through a boil or run fungicide treatment through it. Kokosgoldfish.com has diseases and treatment options which I think would be helpful to you & Kimpy. Perhaps a stress enzyme would help as well. I wish you well with your aquatic endeavors. The fisb certainly need to be in a bigger area soon. I hope the transfer to a pond goes well for you & them. It needs to be ready for this spring, my friend. Give Kimpy my best & tell him to dig down inside himself and really fight to recover and live a good, full life!