I've used Windex and other glass cleaners on my outside glass for well over 25 years without any problems. The trick is to have 2 soft lint-free towels, dampen one with the glass cleaner well away from the tank (another room preferably) and use it to clean the glass, use the second (dry) towel to dry/buff the glass.
If you're still too nervous about that, use a 50/50 RODI/Vinegar mixture... though I found it really doesn't cut greasy fingerprints off the glass like Windex without some extra elbow grease.
HTH