Floaty old common or comet goldfish

[cow]

Hello, I have been having some problems with my OLD (13 Yrs) common goldfish, not the fancy kind, with floaty problems, he will sit at top of tank all day ,, when he poos he is okay, but dont do it enough, tonight he is going but it is clear like a bubble, i fed him for the first time ever today a piece of frozen brine shrimp, will this help, i feed peas alot and it seems he gets gassy and sits at top and list over and back again, this is scary! He gets this about this time every year but lately it has been every day for 2 months, the other night was bad, he ate some gel food and swam fast and jerky, then finally sit at top and kept laying on side, next day after some water changes, I do them every 3 days, he was swimming fine, there are 2 6 inch fish in tank, and it has the following 2 filters rated for 40 gallon tanks, one large airstone, ph 8 nitrites 0 ammonia is 0.25 but it always is, cant get it to 0, i might not be reading my test right, so close to color of 0, i have new filter in my house to remove ammonia and the tap has to be 0 now , since the assured me that is what new filter removes, and the color is the same or slightly different , cant tell, from 0 in tank, the nitrates are 40
The other fish is fine, and has been the whole time, I have tried it all!
Any suggestions, or maybe if you think the brine shrimp is working it will help him, i am hoping,If it does should i give more or go back to regular feeding, progold, coblat flakes, etc
Thanks
Im not sure what exactly the type is for this fish, it is a goldfish but not one of the fancy egg shaped ones, it was originally considered a feeder fish from a very reputable store, Never has been sick!

 

[Wiggles24]

Picture?

 

[cow]

cant figure out how to post a pic from my picture it program?? Sorry