So to help easier diagnose without reading a science book
Mobile nutrients are:
- nitrogen in the form of nitrate,
- phosphorus (P) in the form of phosphate,
- potassium (K),
- magnesium (Mg),
- chlorine (Cl),
- zinc (Zn)
- and molybdene (Mo).
A defecieny or problem (toxicity) caused by any of these will show in the older growth first.
Chlorines an interesting one ... as it shows in the bottom leaves ... a bit like yours.
^^ that's a chlorine toxicity (as our canna won't show a Chlorine Def .. as it doesn't want or need it)
Nitrogen def make leaves go pale
Potassium is little black spots ( i think)
...
I think between science and a "gut feeling after reading all the info" we have a winner ... Chlorie and PH