Hard water is the preferred habitat for certain fish species; unfortunately, it isn't so great for the clean, transparent glass of our fish tanks. If you've ever noticed a white residue forming on the top of your glass in a freshwater aquarium, you're probably seeing the result of the evaporation of hard water. The residue left behind is likely a lime (calcium carbonate plus additional ions) build-up on the glass called "limescale." Saltwater aquariums also can get "salt-creep," which is salt residue on the glass and top of the aquarium.
As mineral-rich aquarium water (hard water) evaporates, the gaseous water leaves behind the heavier elements that adhere to each other and to the glass, leaving an ugly, streaky white residue behind. If you live in a home with hard water you likely have the same kind of build-up around your shower head or faucets. While these residues won't hurt your fish or your aquarium, it can nonetheless make them harder to see and it isn't pleasant to look at spotted and streaked aquarium glass.
Safe Removal of Lime Buildup From Glass
Lime buildup looks so terrible that there can be a great temptation to use home cleaning products to remove it. However, resist that urge, as even the smallest drop or leftover residue from any cleaning agent will likely be lethal to your fish. This rule goes for the top edges of the tank as well.
Products made to safely remove lime buildup from aquarium glass are available. Check your local pet store for fish safe cleaning sprays. If you're looking for a greener and more inexpensive alternative, try plain white vinegar on a dry aquarium. Not only is vinegar a natural disinfectant, but its acidity also dissolves stubborn lime deposits.
For this method of cleaning, you will need to relocate your fish to a holding tank. Once all of your fish have been safely removed, drain the tank completely of water and remove any plants or decorations. Gravel and other substrates can be removed or remain behind if held in place by a barrier to prevent spilling them. Lay the tank down on a towel, and pour enough vinegar on the affected glass to cover it. Let it sit for 10 to 20 minutes, then scrub with a non-abrasive pad or cloth.
If you have a stubborn patch of build-up, try using a razor blade or algae scraper to gently scrape the scale away from only glass panels. Do not use a razor on plexiglass or other types of acrylic tanks as any sharp tools will easily scratch them. Be careful not to damage the silicone glue holding the glass panes together. Once you're done, rinse the tank thoroughly before refilling.
Preventing Lime Buildup
The easiest way to avoid spending time scrubbing your tank is to prevent scale from being deposited in the first place. As the evaporation process is the main driver of this residue, check your tank's water line every two or three days. Low humidity days in winter months or warmer temperatures in your tank water will both drive the rate of evaporation upward.
As the water begins to evaporate, your remaining tank water will become slightly harder (a higher density of minerals per liter of water). For this reason, the best replacement for evaporated hard water is distilled water, if you are only topping off the tank and not doing a water change. The water that escaped was pure, so the water you add back in should be pure water as well. Distilled water has all of the minerals removed from it. Replacing evaporated water with additional mineral-rich water can slowly lead to a lethally high level of some minerals. Using distilled water to replace evaporation eliminates this risk.
However, when doing a partial water change in your aquarium, you should use your regular hard water source to fill the aquarium after removing some of the old water. This will keep the minerals at a normal level and help buffer the pH, preventing gradual acidification (lowering of the pH) of the water. A water test kit that measures pH, hardness (GH) and alkalinity (kH) is good to have to make sure your water quality is safe for your fish.