Distilled pure white vinegar -- It's harmless in small doses. In fact, when you first get used tanks or equipment, you should give it a vinegar rinse instead of soapy rinse. Leaving traces of vinegar is less harmful than leaving traces of soap.
In fact, once in a while, I will soak all my powerheads and pumps in a 50/50 vinegar & water solution. Does wonders! I keep a huge gallon in my cabinets at all times.
Plus instead of window cleaner chemicals, you can put a mix of vinegar and water in a spray bottle to clean your glass, knowing that a little bit that accidentally ends up in the tank is harmless.