I don't know that it's safe really.. but when I was a kid I always cleaned the glass and hoods with Simple Green and never lost a fish to it. I wasn't exactly careful, I'm sure I contaminated the water in at least a few tanks (I was about 7, didn't know better) and it's what I use still- more carefully. It does a good job though, and it's the safest cleaner I know of for birds, small animals, people, etc.