https://ew.com/books/will-smith-memoir-fresh-prince/

Ours (https://web.archive.org/web/20061031125219/http://www.jumptheshark.com/f/freshprinceofbelair.htm) was season 5, episode 15, 'Bullets Over Bel-Air,' (https://freshprince.fandom.com/wiki/Bullets_over_Bel-Air) the one (https://screenrant.com/fresh-prince-of-bel-air-best-worst-episodes/) in which I got shot (https://www.imdb.com/title/tt0582951/reviews) and Carlton started carrying a gun (https://www.eonline.com/news/715795/10-times-the-fresh-prince-of-bel-air-got-real).

I think it jumped before that….

Will Smith Is Wrong About Fresh Prince Of Bel-Air's Decline (https://screenrant.com/fresh-prince-belair-will-smith-carlton-gun-wrong/)

"Bullets Over Bel-Air" may be much more dramatic than normal, and is certainly a departure from Fresh Prince of Bel-Air's usual tone, but contrary to Smith's assessment, most fans seem to regard the episode as being one of the best. Whereas the phrase "jumping the shark" traces back to a storyline involving Henry Winkler's The Fonz on Happy Days that employed unlikely and over-the-top events for the sake of novelty, "Bullets Over Bel-Air" tackled the issue of gun violence, which was far from a rare problem at the time. It does so in a refreshingly grounded way too, with Smith and co-star Alfonso Riberio turning in nuanced, emotional performances.

That said, Smith probably isn't far off as to when Fresh Prince of Bel-Air did indeed "jump the shark". Season 5 had already played host to lots of overly silly, or outright bad episodes, such as "Fresh Prince the Movie," in which Will spins a tall tale to his friend Jazz about how he witnessed a murder and the Banks family was forced to hide out in Alabama under the Witness Protection Program. That's definitely a candidate for shark-jumping. There's also season 5's opening, which undoes Will's decision to stay with his mom in Philadelphia by literally having NBC executives force his return to Bel-Air, which reeks of not knowing how to write Will out of the situation they had placed him in.

Another regrettable part of The Fresh Prince of Bel-Air's penultimate season was the long storyline involving Will and Lisa's romance, which teased a wedding several times, but ultimately went nowhere and felt pointless, with Lisa just fading away in the end. The Fresh Prince of Bel-Air would have a few more genuinely good episodes after "Bullets Over Bel-Air," including the series finale that features a terrific moment in which Uncle Phil declares Will his son, but those were few. Thus, it's arguable that "Bullets Over Bel-Air" wasn't the beginning of Fresh Prince's downturn, but instead its last gasp of true greatness after said downturn had already begun.

What jumped the shark on this show is they didn't mention about both will's college graduation and new job in the west coast episodes.

I think it jumped before that….

The obvious go-to is the say that the show "jumped the shark" (https://web.archive.org/web/20061031122257/http://www.jumptheshark.com/f/freshprinceofbelair.htm) when Daphne Maxwell-Reid replaced Janet Hubert as Aunt Viv. But what about the episode where Will's mom marries Lisa's dad? To backtrack, Will and Lisa decided not to walk down the aisle and get married. So instead, Will's mom and Lisa's dad decided to get married at the altar, which would make Lisa and Will step-siblings. And as strange and odd the whole event was already, it was never brought up again in the series. I mean, don't you agree that it's weird that they are now step-siblings but they used to date and they almost got married?

Janet Hubert Whitten couldn't control her anger management correctly as James Avery does, and that's another reason why both Will and Janet couldn't get along with each other before Janet abruptly departed.

Lisa, her father Fred and Vy, Will's mother, moved back to the east coast before the sixth and final season began.

i think it jumped after series 2, where it became increasingly raunchy and meanspirited after (will tricking his girlfriend into marriage, will cheating on his date with her mother).
i do enjoy the last two series however, despite it never reaching the quality of the first one.