I have similar bouts of depression and I can go up to a week feeling down with random bits of crying in between, it frustrates the crud out of me...
Something I do (instead of impulse buying... which I kind of did tonight... beautiful red/white butterfly that I decided to give to the fiance for his birthday) is carry a small stuffed animal around with me... sound's odd I know, seeing as I'm 22 lol but it does help.
I also just like to go window shopping or just walking around (candy helps sometimes too)
If you really think a second fish would help, go for it, but make sure that you really have a connection with him/her before you buy or else you just wind up kinda "meh" about the whole thing, which may or may not worsen the situation.
Also, if you have a special tea or drink that you enjoy and really happy kind of disney-like movies, a nice way to calm down is just sitting there for a few hours sipping on hot tea and enjoying the movie (my favorite teas are jasmine and camomile)
Hope you feel better soon!!! <3
5 bettas: Шостакович Shostakovich, Dimitri, Blood, Fancy Pants & Miss Priss; 1 cat: Rajah