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Can you use outdoor rocks/gravel for your fish tank

Updated on February 23, 2013

Lots of new fish tank owners wonder if they can start using garden gravel for his or her fish tanks, because this can be an incredibly controversial subject between quite a few owners, you will normally see a mix of responses to this concern, but who is right. Well it all depends on what kinds of gravel/rocks you are using and the way you are going to clean them.

The reason why some kinds of gravel/rocks can be damaging for your fish tank is that they can lead to a change in the pH level and hardness of the tank water, some of these include:

• Chalk

• Marble

• Geodes

• Dolomite

• Shells and Crushed coral

• Limestone

• Tufa

These rocks will drastically raise the hardness of the water mainly because they contain a substantial amount of calcium, which can be harmful for your fish. So you should avoid using these as accessories for your tank. If you're not sure if the gravel/rocks that you want to put in your tank contain calcium you'll be able to test them by placing a couple of drops of vinegar over the gravel/rock,if it fizzes or foams then you certainly do not want to use this rock as it contains calcium deposits.

It is possible to test the gravel or rocks by positioning them into a bucket with the same water that you use with your aquarium, then test the pH level as well as the hardness of the water, leave it for a week and then test it again, if you discover that there is a considerable difference to the pH level and the hardness of the water you will not want to use these as they will likely create problems.

That doesn't mean however that all rocks are unsafe to your tank, some safe rocks that can be used for your tank include:

• Flint

• Slate

• Basalt

• Granite

• Sandstone

• Quartz

You could gather these rocks from an anywhere, but if you are collecting rocks for the tank you'll want to know that you don't know what the rocks have come in contact with, this can be anything from rocks that have been collected from fields having pesticide residue on them, to rocks found in the backyard having traces of weed killer on them.

The best place to gather rocks could be near a healthy body of water such as a clear stream, river, or shore. These are great places you can find rocks because the rocks that you will find in these places will have already been soaked and would have their toxins removed.

Even though these rocks will be safer for your fish then the rocks that include calcium deposits, you will want to make sure that they will not contain any water based problems such as parasites, algae’s or other diseases. You can do this by giving the rocks a thorough wash, if you wish to be even more thorough then you can give them a quick boil.

Make sure that you avoid any rocks that have any lines, colours or veins as these might be metal ores that may be toxic to your fish. Also avoid natural lava rocks as these also normally contain large amounts of metals and toxins, that being said the only expectation will be quartz veins that will be white in colour.

Remember when collecting rocks you'll always what to make sure that they do not have sharp points because might cut the fish.

If you would like to learn more about fish tank care, please visit


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