coopercat -- I'd submit that the only thing you are doing "wrong," is that you may be mistaking the amount of time it takes to learn the "nuances" of the Equinox's "dialect." It's EASY for ANYONE to hear a good solid tone, see a solid "25" on the display, and dig a 2-inch deep penny, of course. But, that's like saying you "know Spanish" because you have learned that "adios" means "good bye." The HARDER part, as I am sure you know, is becoming FLUENT -- i.e. learning the nuances of the unit's language. There is of course no shortcut; not sure if you are new to detecting, also, or just new the Minelab and/or the Equinox, but bottom line -- it takes hundreds of hours and thousands of digs to gain "expertise" -- and it's that expertise, which you can only acquire gradually over time, that slowly lets your number of "junk" digs to decrease, and your number of "good" digs to increase...i.e. the "trash-to-treasure ratio." Your ears and brain need time to learn to decipher the nuances, such that you will dig less of the iron high-tone "false" signals, and more of the high-tone "good" signals.
And yes, "15" is way too high for a nickel, with the exception of a few of the 35% silver "war" nickels. A nickel, as mkus noted, will be in the 12-13 range, possibly as low as bouncing between 11 and 12 when really old/really deep. And of course, that is ASSUMING it's a clean target, in low-mineral ground, with no other targets nearby that might throw things off...
Steve