While this doubtless already is adapted to European standards, spicy is hot enough for most people unaccustomed to Indian food. Note that Indian food in general is quite expensive here in Germany when compared with most other restaurants. Usually you'll get very good food, though. Please look at the price comparisons with this in mind. I tested all these restaurants and would recommend the Taj Mahal, though with the exception of the Indian Palace all are very good.
All restaurants offer all kinds of meat, fish and vegetarian dishes.
Specialized on Tandoori dishes. Good food but small selection of dishes. Most dishes are quite affordable even on low budget. Indian Palace Right in the Center of Heidelberg, near Marktplatz, serves a wide variety of Indian food, though the food's quality is not up to the standards set by the other restaurants. I've been there twice, but won't go there again.