I use a red lense on my flashlight as well, but I'm not so sure the inhabitants don't see it. It may be more along the lines of it not bothering them. My brittle star definitely doesn't react to it, so it may be true in his case. My shrimp don't seem particularly bothered, but they do react to it. Same thing with my clownfish, and if I shine it directly at her, she definitely sees it, and doesn't care much for it. All that said, I'd say a low intensity red light would be perfectly fine to use at night. I've even used really dim white light for a night light. As long as it's dim enough, it's not going to bother any of the tank's inhabitants.