you cant lower the general harndess of a saltwater aquarium. Why would you even want to?
have more fishes and plants.
There are two ways to help control water hardness: use a packaged water softener or use a mechanical water softening unit. Packaged water softeners are chemicals that help control water hardness.
Yeah.... uh.... get a freaking filter!
The best way is to control the harness and softness of the source water for the aquarium. If you need soft water, then use a water softener or reverse osmosis machine to lower the water hardness. If you need your water harder, then you can purchase remineralizing compounds that you can mix with water change water and then add to your tank. You can find remineralizers at www.bigalsonline.com. Look under Water Treatment > Ph Regulators & Buffers. I recommend Kent's R/O Right.
If your aquarium is getting lower in water, it means the water is being vaporized by the light and no need to worry, just refill it.
If the calcium is a result of pool chemicals, draining some or all of the pool water will lower the calcium hardness level. If the cause is the fill water, commercial hardness reducers or chelating agents will bond with the calcium to keep it trapped in solution.
If your pool kit tests for both acid and alkaline pH, General Hardness, Ammonia, Nitrite and Nitrate then it will do the job. If not, then you will need to get whatever bits are missing if you want to keep tabs on what is happening in your water.
There are crystals available in many pet shops that can be placed in a filter to remove hardness. To increase hardness you can add sea shells. To neutralise the water you can get 'Neutralising blocks'. If you are really keen you can use a reverse osmosis filter to remove all the hardness (and other impurities) from the water and then add all the required chemicals to make the water exactly what a specific species requires.
Water hardness or softness refer to a number of water parameters. Most commonly hardness refers to what known as General Hardness(GH), which is basically a measure of the calcium and magnesium salts present in water. There is also Carbonate hardness(KH), which is a measure of the carbonate concentration of the water. Combined, these two form Total Hardness, ie, GH + KH = TH
In general, the hardness of municipal water has little effect on the color of hair dye or on how long it lasts.
The general rule for stocking a fish tank is 1 inch of fish per gallon of aquarium water.
Water changes are the quickest and easiest way to lower nitrates in a marine aquarium.
Magnesium sulfate (MgSo4) does not cause hardness of water.