Arimidex decreases circulating concentrations of estrogen in the body. This is what is necessary to prevent the estrogen related water retention. Another major factor is your diet. Specifically, a high sodium intake.
I have never know anyone who has experienced a loss in gains but for those who are concerned, a combination of Arimidex, Nolvadex and diet adjustments may provide the best results. More specifically, a lower dosage of Arimidex without affecting your strength/size and to decrease circulating estrogen levels, Nolvadex to protect against the elevated estrogen, if any, from binding to the actual receptors and an adjustment to your macronutrient ratios to minimize the water retention related to your diet.
Jenetic