While it's true that too much physical interaction with a fish is not good for it, the occasional touch won't harm it. And I think as long as you have made sure you don't have lotion on, it's fine to stick your arm into the water. The amount of oils from your skin that would come off would be minute in comparison to the water volume. If you are really worried about that though, you can always try these:
They go all the way up past your elbow, almost to the shoulder. I got a pair for when I need to handle my electric catfish and they work great.