You can use white distilled vinegar and an aquarium scrubee. The vinegar is safe once it's dried it's gone, but don't get it in your water.
try dropping the waterline and scrubbing it off with the vinegar.
if it comes to the worst, I got this really nasty water ring off the top of a tank once using 90% rubbing alcolol and a ton of salt. the salt makes the alcohol abrasive without scratching the glass. but that tank was empty. again the alcohol leaves no toxic residue behind.
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