A reader asked about finding white residue on aquarium glass:"We have really hard water, and my aquarium developed a white deposit all around the top part of the glass. I tore the tank down to give it a good cleaning, but I can’t seem to get the white stuff off. Any suggestions?"
It sounds as if you have what is often referred to as lime buildup on the glass. This occurs when the water has a lot of minerals dissolved in it.
As water evaporates, it leaves behind the minerals, which adhere to the glass, leaving the ugly white residue you see.
It looks so terrible that there is a huge temptation to use home cleaning products to remove it. However, resist that urge, as even the smallest residue can be lethal to fish. There are products, such as Safe Aquarium Cleaner, that are made to remove lime buildup on aquarium glass.
However, there is a readily available, inexpensive alternative – vinegar. That’s right; plain old vinegar. Lay the tank down on a towel, and pour enough vinegar on the affected glass to cover it. Leave it to sit for 10-20 minutes, then scrub with a non-abrasive pad or cloth and rinse well. You may have to repeat the process a couple of times for each side to get it all off.