Calcium buildup, also called calcium creep or hard water stains, can detract from the look of aquariums. In the saltwater side of the hobby, the calcium crust can also contain salt crystals, making it even more of a pain. As ugly as calcium buildup looks, you can remove it with regular work and natural cleaners like vinegar.
If you have a small piece of equipment or decorations you can remove from the aquarium, you can remove calcium buildup by boiling it. Adding several teaspoons of vinegar to the boiling water can enhance this technique. Take care -- boiling could accidentally bleach some decorations and damage some submersible equipment like powerheads. Keep in mind that sudden temperature changes can crack glass, so avoid using this on glass equipment. Additionally, use caution when working with hot water.
On stains outside of the aquarium, or on removable aquarium equipment, you can use vinegar to remove calcium buildup. Simply apply the vinegar to the rag and wipe away any calcium buildup. For stubborn algae, use this technique with an aquarium algae scrubber instead of a rag. However, never use this technique on the inside of an active aquarium since vinegar contains an acid that can lower pH and cause issues with water chemistry. Never use an algae scrubber you've doused with vinegar, and thoroughly rinse any aquarium decorations or equipment before returning it to the aquarium.
Regular maintenance helps keep calcium buildup at bay. It takes less effort to remove one week's worth of calcium stains than it does to take a month's worth. The longer calcium deposits sit, the harder they set up and the harder they get to remove. Ideally, you should wipe away calcium buildups bimonthly. Some aquarium hobbyists wipe away calcium buildup when they do their bimonthly water changes or other routine chores to establish a routine.
Buildup Inside the Aquarium
Sometimes, calcium deposits can build up inside of aquariums. This can happen due to chronic evaporation issues and hard water. You cannot use vinegar inside of the aquarium, since it might harm fish. You have two options to remove buildup inside the aquarium. With a clean -- and vinegar-free -- algae scrubber, you can wipe away at the buildup. It will take more effort, but you may be able to remove the buildup with elbow grease. However, for stuck-on deposits, you may need to strip down the aquarium and clean the inside with vinegar. This process will stress fish out and may cause issues with biological filtration.