Ammonia level - normal
Nitrate (NO3) - 0
Nitrite (NO2) - 0
Hardness - 60ppm
Ph (in tank) - 7.3
Ph (tap) - 7.7 Note: I always put a little bit of Ph minus in the tank in order to lower the Ph to the level my fish are used to

Tank Size - 15 gallon running 5 years
Name of Filter - Top Fin size - for 20 gallon

I change 30% monthly
Fish - one goldfish
Water stuff - only stress coat to remove chlorine and a little bit of aquarium salt
New fish - none
Feed - goldfish flakes from Wardely
No ick or velvet, no fungus, spots or bloody streaks, his scales are not sticking out
Unusual Behavior - he will simply float around the top of the tank not swimming, he eats but rarely leave the surface, but does no gasp for air...

I have no idea what is wrong with him, but this happened a couple of months ago in a different tank and the goldfish died so I am really worried

Can you please post the actual number for the ammonia? I also see that you do not have any nitrAte, telling me that the tank is not cycled. I also noticed youre only doing a water change once a month. I would recommend upping that to once a week, 30% is a good amount to remove and make sure to add a dechlorinator(such as Prime or Amquel Plus) when adding new water.

Now, this sounds like it MAY be SBD. I would try fasting him for a couple days and then feed one pea with the skin taken off on the 3rd day. Flakes add to the problem as they float, causing the goldfish to gulp in air along with its food.

Someone else will chime in with an opinion soon.

From your information,I am in agreement with Glitter as I think that you need to do weekly water changes on a regular basis.
What test kit are you using to get those results on your params? A tank set up for 5 years,normally would have some sort of Nitrate reading unless you bumped your cycle somehow.
if your tap water normally is 7.7 and you say that you add "a little bit of Ph minus in the tank in order to lower the Ph to the level my fish are used to ", why not try more frequent water changes to keep those levels more level/even?Changing your ph and having swings can also cause problems for your fish.
I would also look into a better filter.Goldies normally need at least 10 times the filtration of their tank.Now its been quite a long time since I've used a Top Fin,but I think that your fish may need a bit more powered filter.Do you keep any airstones in the tank at all ?
Normally a fish hanging at the top,is not a good sign.It can be swim bladder,but it can also be not enough oxygen going through the water,infection,or often times parasites.

Do you have aeration in your tank? the fish need oxygen and also, stress coat does not remove chlorine. I use amquel plus. You need to provide lots of aeration when you use these chemicals. you can add a bit of aquarium salt also to provide some electrolytes. follow the directions provided. you also need to change your water weekly. 25%

Any updates?