well said km,
nitrix or any NO product doesn't increase water retention or movement into the muscles, it actually is the opposite, it increases blood flow and count to promote the pump, and in doing so it changes the water:blood ratio in the muscles, the more blood you have in there the better because it is bringing in more nutrients into the muscles = muscle gains.
What I think you are thinking of is creatine, which, in the case of monohydrate creatine, just swells the muscles with water and creatine which gives you that bloated look. However, with creatines like ethyl ester styles, it uses sugar or fat cells i believe to penetrate into the muscles, which is more efficient and does not cause the bloat like mono(essentially 5grams of cee = 15-20 grams of mono). Now some people like the bloat, personally I don't and I like the feeling of cee, not to mention if you take mono and then stop you lose that extra water weight and shrink more than you would on something like cee.