The acid in citrus, like limes, lemons, or oranges, works wonders to break apart the water stains on glass. And it is non-toxic, so you should feel comfortable using it. I will clean the entire cage's glass when I do a deep clean (along with a bit of bleach to disinfect), but sometimes I just get irritated with how the doors are starting to look and clean those quickly when I can. Especially if I am expecting guests so that they can have a clear view into the cages.
Ugh! Water stains that will not come out with water, soap, or even harsher home cleaners.
The same door panel after scrubbing with the lemon and wiping it off with a wet rag and then a dry rag. If I had spent a couple more minutes I could have gotten the sides to look just as impeccable. But in a quick 5-minute effort the front of the cages look like new. And now the geckos have a fresh canvas upon which to defecate, leave finger-paintings, and otherwise make a mess again. Case and point: