I've been getting very moody and tired lately. because depression is chemical, can it be caused by an unhealthy diet also?

Not really: It is true that depression has a chemical basis but the etiology is more complicated the link between certain food items and depression is reported but happens rarely and is controversial.

Possible factor: Depression does have a biologic component, but some studies also show that dietary factors may be an influence. In one, diets of vegetables, fruit, meat, fish, & whole grains were associated with lower odds for major depression/dysthymia & anxiety disorders; "western" diet of processed or fried foods, refined grains, sugary products, & beer with more likeihood: http://tinyurl.Com/mxlmfg8.