I can’t really say, fatigue is a common symptom of heart problems, but it can be a symptom of a number of other things. As heart problems progress you’ll start to notice that they breaths with their mouth open after slightly exerting themselves and that their abdomen will start looking swollen with excess fluid retention.
Laying down a lot more than the others can also just because of personality. I’ve had a gander who passed away from heart issues, I’ve seen similar fatigue and fluid retention from pancreatic issues in another gander, and I also have a gander that has always just been sleepier and more relaxed than the others since the day he hatched, it’s just his personality, so sitting around more than the others can be cause for concern, but it also might not be an issue at all. It’s hard to say.