So I'm kind of pissed off because about a month ago some substrate got caught inbetween my mag float and put a couple nasty surface scratches on my tank.  I can't feel them, but 3hrs of buffing with cerium oxide barely took out the scratches.  I got some 2000 grit sand paper and buffed for about an hour and they were noticeably lighter but still aren't gone.  I have some 600, 800, 1000, 1200, and 1500 grit coming in from ebay to finish off the job.Â

The real reason I'm making this post though is that I noticed that buffing with 2,000 grit didn't add any visual scratches to the glass, so for the past week I've been cleaning my aquarium glass with a piece of 3,000 grit sand paper sandwiched between my mag float.  I can SLOWLY move tha mag float across the surface of my glass and remoce ALL of the debris in 10 minutes (I don't have to circle 20 times in the same spot to get rid of hard algae spots).  I just thought others might be interested in trying this out if they're having trouble with glass algae.  I can even get into really tight corners using this method.  I suggest trying it in a corner of your tank first to make sure it won't damage your glass.  Mine is tempered and there's no effect.

Thanks for the idea. I have a used tank with tempered glass and was wondering how to remove all the little scratches. This will help a lot. Do you leave the sanpaper in the tank or do you take it out between cleanings?

I take it out.  Sanding scratches is a pain in the ass.  Just don't scratch your tank   I usually start with 1000 grit to rough up the scratch and then smooth it with 1200, 2000, 2500, and 3000.  Then I buff out all of those micro scratches with cerium oxide.  It's about 5 hours of work of a 5" scratch...

I cleaned up a few more scratches this weekend. I tried a new technique... I roughed up the area with 1000 grit until the scratch didn't catch light anymore and then buffed it for 30 mins with cerium oxide. No more scratch and crystal clear glass. It sucks I can't reach the lower ones (I can't empty the tank unfortunately). Maybe when I move the tank I'll be able to buff them out when there's no water in it Hopefully this helps some other people that have scratches pissing them off.

Here's what I did:

Lowered the water line below the lowest scratch I wanted to get out
Covered the water with plastic and taped the borders to prevent cerium oxide from getting in the tank (It's non-toxic, but still a good idea to keep it out - Lethal at 50g/kg according to the MSDS)
Roughed up the scratch until I couldn't see it catching light
Buffed it out with a semi wet slurry of cerium oxide with a 6" felt buffing wheel (You can find these at menards or lowes)