He has had swim bladder issues in the past and feeding him peas has helped. But those times he floated upside-down and kind of bobbed around the middle of the tank. I’ve never seen him float on his side at the bottom before. He is either unable or unwilling to eat peas now. I’d really like to help him if I can.
How are the water levels (ammonia, pH for example)? Also temp?
I’m sorry to say it looks not good, but if it is a reaction to something environmental, you may be able to correct.
If you don’t have all the test strips, take a water sample down to your nearest pet store and get it tested. PetSmart or PetCo if you don’t have a local place. Failing that, try an aggressive water change tonight (say 50%). Do you use any aquarium salt with your goldfish?
ETA : if there are other fish in the tank, are they acting distressed? If there are other fish, can you put goldfish in a separate tank?
pH between 7.2 and 7.8
Alkalinity right on 120 (ideal)
Hardness between 75 and 150
Nitrate - between 0 and 20
Nitrate - a kind of greenish beige - same color it’s been for 4 years - definitely no hint of pink to it. I’d guess it’s safe since all the caution to bad colors on the bottle have a definite pink tinge.
Temp is on the low side where it has always been.
Filter was changed about 2 weeks ago.
Pump is working.
Basically, my water is exactly what it has been since I got him 4 years ago (shitty tap water - he seems to love it).
There are no other fish in there. He has been alone since 9/2010 (yes, there is a reason why I remember this so specifically).
His first time with swim bladder problems was in 10/2010. He was only tilted to the side a bit and didn’t seem to be able to make it to the bottom. He’s had buoyancy issues probably 3 or 4 times since then and peas always worked. He has never looked like this before but when I search google it seems that settling on the bottom, completely on one side is much more common than what he’s been doing.
I actually hand fed him some mushy peas (had to open his mouth). I didn’t see him spit it out so either it’s in there or he’s holding it in his mouth.
Ok - if the water readings really haven’t changed then hope peas get him through it. If they do, try feeding an orange slice occasionally. You could try it now, but he’s not looking perky enough to take interest. The orange slices help with swim bladder issues, and the goldfish seem to love them. I have better luck with horizontal slices rather than wedges. You can also experiment with things like lettuce. The basic idea is to be proactive about getting him roughage to keep the bladder working properly. If he likes peas, great. I had mixed results (some people say they need to be peeled, bleah) and my fish didn’t seem to like them. Love oranges though.
Goldfish also like a little salinity, which is why I asked about the aquarium salt. That might be worth adding tomorrow, if you can can get some. It sometimes perks them up a bit.
Any discoloration at all?
Likely is a swim bladder issue, but when you say filter was changed what exactly do you mean by that. You filter contains a lot of the beneficial bacteria that converts ammonia to less toxic forms. If you change out the media all your bacteria goes with it and this can cause an ammonia spike. Doing something like daily 50% water changes would help if that was the case.
As alluded to earlier, goldfish are omnivores and regularly need roughage as part of their diet. Flakes tend to be heavy on protein, so you really should supplement with fresh veggies and fruit.
It was just a regular filter change because it was coated in algae. I don’t do water changes around filter changes to make sure I don’t remove too much good stuff.
Anyway, I’m sad to report that after 4 years, Mr. Vimes is no longer with us. 
Sorry to hear that. 4 years is a good run for a fish. My longest was a betta who lasted nearly 3.
I had a rescue goldfish. I was cleaning out an apartment and put three little goldfish in a bag full of water, in the hot car while I did errands all day. Took them home and put them in our big tank. Two of the three died within a week, but one lived and had the place to himself, and grew huge. As big as my hand, a very pretty fish. One day he died from what I think of as exploding goldfish disease, but he thrived in there for at least two years.
His body was as big as the palm of my hand. I buried him in the back yard (after dropping him about 4’ to the floor - he went plop )
I’ll definitely be getting another fish but it wont be the same kind of goldfish. I think I may get a comet or common. Our tank is 30 or 35 gallons (can never remember) and I’d like to get a goldfish to live for a really long time and get really big. I need to clean the tank and start over though because I’m not sure what killed Mr. Vimes.