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This is a common error I notice foreign speakers of Japanese making, especially if their first language is English. In American English, it's perfectly good etiquette to just say "Thank you," to your waiter or the tenant at the bank, but this doesn't translate to "Arigatou". Arigatou on its own when directed at a stranger sounds sort of weird, to be honest.

There are various ways to say thank you, and different situations call for different words. For example, after a lesson, you should never thank your teacher with "Arigatou", but be more polite and use "Arigatou Gozaimashita". You are not only showing your appreciation for the service you have received, but demonstrating your respect for them.

In any situation, erring on the side of politeness is preferable. So "Domo Arigatou Gozaimasu" or "Arigatou Gozaimasu" are almost always 100% safe.

Rule of thumb:
Very casual situation in which you are of higher status because you are the guest of an establishment (as in when patronizing a store, restaurant, bank/post office/etc), "Domo" is a warm, friendly, and brief way to give thanks.

In a situation where you are of equal status (as with friends of the same age), "Arigatou" is just fine.

When you are of lower status interacting with someone who has greater authority or is older than you, you should almost always use "Arigatou Gozaimashita", or for extra politeness, "Doumo Arigatou Gozaimashita".


PS- As for the difference between Arigatou Gozaimasu and Arigatou Gozaimashita, I don't have 100% confidence on this, but from my experience it seems more natural to use "Gozaimashita" when the favor you recieved has either been completed or is at a stage of inevitably being completed. Gozaimasu seems more natural when you have been promised a favor or the favor has not yet come to pass.

I haven't done a lot of actual research into this so feel free to correct any mistakes.

thank you mrgreen

Also, it seems like past tense is often used as set phrases by servers or people working where you are patronizing. It seemed to me that store people always said, "Arigatou gozaimashita!"

Similarly, there is a phrase that waiters/waitresses use when they take your order or when they acknowledge that you are waiting for them: "Kashikomarimashita!" It generally means either "I understand your order and I'll get right on it" or "Give me a minute and I'll be right with you." I've heard it used both ways.