True some fishes do stress out with lack of decorations however, give it time they get used to it. Majority of cats don't come out when you have decor because they prefer coverage. They hunt for food at night or when there's low light and hide when there is light or the light comes on. From my experience only I've never had any cats stress out on me I never use decor and they are always fine. I've had my jar and a few tank mates freak out on me for a couple of days when I took the decor out in his tank and everytime I turned the light on he 'd get skittish however after a week or two he was fine. He doesnt' even mind it when people walk by or place thier hands on the glass(mostly the kids) because they got use to it. However, I will say that the dats might go at it. As they are bullys amongst each other the bigger or more aggressive one always bullies the other dat(s). At younger and juv stage anyways.
I'm not suggesting not decor though just not the huge driftwoods and slabs of rocks that cats can dissapear to. Some fake or real plants and a solid rockthat the cats can retreat to or still go to to feel safe but not to where it literally disapears or hides itself completely. Although my don't have huge decors mine use the filter tubes to squeeze between or the corner of the tank. Some of mine raphaels use to use the fake plants until they got to big. They still lay agaist it from time to time. From my own persoanl experiences the cats are more active this way. Just one way to get the cats active.