Rosh Hashana 18a Rambams opinion

18a.  Av mipnei Hataanis.  Rambam held (see PHM here) in second Mikdash they fasted Tisha B'av.  Others disagree.  Huchpelu bo tzaros. Tosfos (R"H 18b s.v. Ho'il) asks why is 17 Tammuz different than 9 Av, that on 9 Av we must fast even if there is Shalom but no Gzeiros, but we can choose if we want to fast on 17 Tammuz, but if many bad things happened also on 17 Tammuz we should have to fast? See the two answers of Tosfos. I think the Rambam understands Huchpelu bo tzaros to be discussing the time of the second Beis Hamikdash, 9 Av was already huchpelu - the gzeira in the midbar and the first destruction.