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You will need to test the PH, KH & GH to find out whether your water is soft or hard. If your PH changes easily in the tank, then you can add crushed coral to help it rise and be stable, or peat moss to help it lower and be stable
It is important for a fish aquarium owner to purchase a ph probe for their tank. It helps measure the pH levels in the water. Certain types of fish will only survive in water with a certain pH level and one would need to know what their tank pH level is for their fish to survive.
usually there is a kit you can get for aquariums There are things available called pH Buffer Blocks. You simply drop one in the corner of the tank so that it gets a bit of current from the filter inlet or outlet and it will keep the pH steady.
check wiki
To lower the pH of 100 ml of water to a pH of 5.5, it would take only around a drop of lemon juice. It is estimated that in a gallon aquarium, two drops of lemon juice can lower the pH by .5.
I would not advise messing with the pH in a salt water (marine) aquarium. If you follow the directions on the particular testing kit that you have you should be OK but messing around with pH is a very dangerous thing to do and may quickly and easily result in dead fish.
Buffers are important because they control the PH or "acidic" balance.
PH can be easily changed. Many water sources around the world usually come from alkaline or hard water. In order to increase pH, you could use chemicals or put crushed rock and shells to make the water alkaline. In order to decrease the pH, you could also use pH, or inject CO2. Another way of decreasing pH is to add drift wood to the aquarium, though driftwood tends to make the water color tea like.
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.
There are a couple of things you can do to control the pH of the water for tropical fish. You can for example drain the water and start over.
Install an efficient pump and filter set. It is recommended that you use gravel on the base of the tank since, it is a natural purifier. Depending on the colour and dirt of water and the activeness of the fish, the water should be changed regularly.