There are three major types of problem with glass that require different approaches to cleaning. This article discusses them and way to address the problem.
The easist problem to solve is the simple case of white water spots on the aquarium glass. These are generally caused by water splashing on the glass while changing water; the drops of water evaporate and leave behind the residue from the dissolved solids - usually carbonates of calcium and magnesium. These are very easy to remove, just use some pure (ie, distilled) white vinegar and a lint free cloth. The vinegar is a reasonably strong acid when used undiluted and will dissolve them more or less instantly. Wipe on, wipe off, they're gone.
What you use for a cloth is fairly important. You absolutely do not want to use paper towels; they leave very very fine scratches and over time you will scratch your tank using these. Plus, they tend to shred a bit and you'll notice microscopic bits of paper on your tank that generally only shows up when you photograph things. This can be a big disappointment when you go to a lot of trouble of cleaning everything to take some pictures and the come out looking like it's snowing.
Hence the recommendation to use a lint free cloth. Be aware also that the common practice of using "dryer sheets" in home dryers imparts silicone (not the sealant, the lubricant) on cloths which can leave a haze. This is well known in show-car circles where silicone is a larger problem when trying to impart say, a concours show quality finish on your Mercedes, parts of it will not take wax well where silicone has been.
The thing the car guys use is cloth baby diapers, and these are kept separate from the regular household laundry and washed by hand with as few chemicals as possible and air dried either indoors or out.
Vinegar is also great for just cleaning the glass even if there isn't water spots. It will get it as good as new - it removes fingerprints, nose prints - basically anything on the glass that should not be there.
Some people use window cleaner, which is made from ammonia, alcohol and blue coloring. This works but is not the best idea. Your really don't want ammonia and alcohol in your tank, they're both extremely toxic to all animals and while plants can use ammonia, alcohol will not do them much good. Ammonia vapour given off when it dried can and will dissolve readily in your tank water. Besides, window cleaner is considerably more expensive than vinegar and of course vinegar is non-toxic (in small doese) to anything living in water in your tank. In fact vinegar can be used on a cloth to wipe down the inside of a tank to remove carbonate deposits from the waterline left by evaporation. It also kills algae if you have some growing at or above the waterline or under the lighting canopy. Some people add vinegar to very hard and very alkaline tapwater to bring the pH down. It's very very safe for aquarium use.
Warm vinegar works better than room temperature vinegar, but for regular water spots room temperature is fine. Vinegar is also the best thing to use for cleaning glass lids and covers - safe, cheap and effective.
Sometimes fishtanks are just coated with white carbonate buildup. This can happen with a tank that's been set up for some time or when you buy used tanks - frequently they are just coated with the stuff. Vinegar works here, but, it may work rather slowly and in this case it's best to heat the vinegar. The warmer it is the faster the chemical reaction of the vinegar dissolving the carbonates happens. Of course if the vinegar is too hot it'll crack the glass if you use enough to very quickly raise the temperature of just the part of the glass you're cleaning. An example of this would be if you microwave some vinegar, get it boiling then throw it in a room temperature tank. Expect the glass to crack from the near instantaneous increase in the temperature of the glass. But, this is in reality seldom a problem as any vinegat hot enough to crack glass is going to be very painful to touch. It's just too hot to work with. So, the rule of thumb here is you want to heat the vinehar just hot enough that it is not uncomfortbale to put your fingers in, but just. Any hotter and it will burn you. This is somewhere in the 110-120F or 50-60C range.
If you have a large number of tanks and want something qucker use hydrochloric acid (chemical formula = HCL, also known as muriatic acid). BUT! This requires several warnings and frankly it doesn't work that much better than vinegar for most purposes. But for very very thick desposits you want to get off very quickly this is what you would use. I CANNOT WARN YOU ENOUGH about this chemical. It is NASY. DO NOT EVER use it indoors, even with proper ventilation it is STRICTLY for OUTDOOR USE ONLY! Yes, an AM shouting.
If you are familiar with handling stong acids - and know that you NEVERM add water to acid, always add acid to water lest it literally explode spraying acid everywhere, then you probably are already aware of this. If not, please don't use it and use vinegar insetad.
HCL is also extremely hydrocsopic. It gives off a vapour that is extremely soluable in water and it will very quickly dissolve in any water that's nearby. Your skin, eyes and lung tissue are all prone. One lungfull of HCL vapour and your know what I mean. You can be easily blinded by this stuff.
All warnings aside it will cut through the nastiest thickest carbonate deposits instantly. Keep in mind it was until about 1976 or so, listed in the Guiness Book of World Records as the strongest acid in the world. It deserves a very, very healthy respect.
It's available in hardware stores and is usually sold as a driveway etcher. Around here it comes in plastic bottles sealed in a thich plastic bag and is kepts in the back room. They will not stock it on metal shelves. It's just nasty stuff.
It can also be found in dilutee form in sxtra strength toilet bowl cleaner and "lime and rust" cleaners in better hardware stores. I would be very leery of these and do not use them. The former contains wintergreen oil and who knows what else to make it the viscous blue liquid it is while the latter also contains sulfamic acid and chemicals that prevent the hydroscopic nature of HCL from manifesting itself. It foams like soap when you wash it with water. DO NOT use these.
After you've cleaned badly deposited glass of the carbonate you may see what looks like a metallic film on the glass. I have no idea what it is, but it may well actually be a metallic film that's built up. It's appearance is rainbow like and while it is possible to remove it with a new single edged razor blade it has taken me one hour to remove one square inch of this stuff. It's increadably tough. I have not yet found a way to remove it. Suggestions have been made to use glass polishing agents such as cerium oxide and optical rouge. The cerium ocide is a bit nicer to use as the rouge imparts a red tint to things as it site in microscopic scratches. Gary Sutcliffe writes:
You also need a lap. This is a semi-soft material that the polishing agent can stick to (on the microscopic level). I used pine pine pitch. Since you don't need optical quality, a soft rag would probably work. The exposed edges of the polishing agent essentially shave the glassJerry Durand added:
I used to maintain several industrial cutting Lasers and had the problem of cleaning burned on crud from the VERY expensive lenses. I found using 1 micron diamond powder in an alcohol base would clean ANYTHING off the glass without making any visable scratches. Just spray it on and rub it around with your finger. It wasn't very expensive the last time I purchased it. I was buying this from Edmund Scientific, but don't see it in their current catalog
Richard Sexton, March 2005