Little leaf on Phal has started to turn black

[rbarata]

15º C it's too low. You should also consider that, since the insulation is not good, probably you have drafts of cold air, especially near the windows (and the air near the windows is also colder than the rest of the air in the room).

See my previous post for my sugestions.

Probably not the cause but what type of fertilizer are you using? The brand you've mentioned has at least 3 formulations (6-30-30, 20-10-20 and 20-20-20)?

Do you know the concentration you're using?