Acne can be caused due to a number of reasons. I feel the most common reason is due to any fluctuations in hormone levels. That is why acne is very common when you first start a cycle or when you come off. However just a high level of hormones such as e2 (estrogen) can result in bad acne. AI's are excellent for keeping your estrogen in range so a must when taking high amounts of test.
An AI will also help with water retention. Just be sure to take a reasonable amount but nothing too big as I feel that can effect your performance (results). The key is balance so you feel good, joints are lubricated and well you know the rest. My favs are aromasin or arimidex... both are great... arom is more lipid friendly so I have that first choice but they are pretty interchangeable. Make sure you drink alot of water as that will help with bloat and help flush out toxins. I suggest taking 2-3g vitamin c daily for anyone who has trouble with water retention.