Basically どうもありがとうございます is the present tense. どうもありがとうございました is the past tense. I know people don't use past tense for saying "thank you" in English; but, in Japanese, we use both those tenses for saying thank you. For example, if someone gives you a free ride to somewhere, you would say "ありがとうございます” when you are riding on the car, and you would say "ありがとうございました” when he drove you to the place, or when you are getting off from the car. Because you are saying "ありがとうございました"(past tense) for what he just DID(just had done) for you. This is the difference between "Japanese ありがとう" and "English thank you." In addition, There is no relation of politeness between those two expressions.

jaja,casi no hay diferencia entre ellos,no te importa nada.

どうもありがとうございます - domo arigatou gozaimasu どうもありがとうございました - domo arigatou gozaimashita Both have the same meaning "Thank you" but the ony difference is that the second one is very formal (as far as I am aware of ) Now from what I understand is usually "gozaimashita" is said/used when someone has done something for you (like a favor or helped you in something) and you're thanking them. As for the first one "gozaimasu" it is said generally. And domo is added for politeness. Hope someone clarifies what I said since I'm not entirely sure I'm right; after all I myself am a beginner in Japanese.