Hello and welcome!
NO! That is not normal for a fish! Fish don’t get overweight from eating too much — they do have metabolic problems, but not the size.
Something is wrong with this guy/gal! The things that come to mind are egg binding, tumor, or dropsy.
What do the scales look like? Are they sticking out, resembling a pine cone? Is the swelling the same on both sides of the body?
NOW, FOR THE GOOD STUFF: PLEASE GET BACK WITH US:
- what size is your pond?
- how many and what sizes are your fish?
- you said it was “an established pond,” but how long has it been up and running?
- what kind of filtration do you have?
- have you recently added any new fish, or made any changes to your pond?
- PLEASE CHECK YOUR WATER PARAMETERS (WITH A LIQUID DROP TEST KIT) AND LET US KNOW THE NUMBER RESULTS OF THE FOLLOWING:
- AMMONIA
- NITRITES
- NITRATES
- pH
- KH
- GH
- WATER TEMPERATURE