The first sentence is active and the second sentence is passive. In the first (active) form you are showing that the treatment is important (in the conversation), but in the second, it is more important that the level decreased. But, yes, they both ultimately mean the same thing. We use passive when the person or thing doing the action is not so important, or it/they are not known. For example, you hear it a lot on the news, when people are not exactly sure if what they as saying is true...e.g. it is said that apples are good for you. Hope that helps

Decrease means to make or become smaller or fewer in size I'm not sure on this one, but i think the sentence would be better like this = Sugar level in blood was [to be] decreased due to the treatment. So basically saying the amount of sugar levels need to be lowered :)