Yeah i say same thing Overwatering, rule thumb yu cant water too much but too often so when yu water, water it good and let then dry well, very well but not too much!
it looks good, but it might look more worst if yu keep overwatering. there maybe might be heat or temp prob's too, but not so bad because plant looks fine no yellow....
dark color mean your soil is over 5.5ph like something 6-8, if your plant looks bright green, little yellow then your ph is low like acid. hope this helps!