Shells and limestone in fish tank - The Planted Tank Forum
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post #1 of 6 (permalink) Old 10-04-2015, 01:24 AM Thread Starter
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Shells and limestone in fish tank

I asked around and tried my very hardest to find out if shells and limestone are ok for the fish in aquariums.

My husband put a medium sea shell my bro got for me from south america on the beach in my 20 gallon. I cleaned and scrubbed and soaked and disinfected it before it went in. I have had my tank for 6 months and do regular water changes. Also it is community fish tank. Mostly mollies. All the fish are healthy and active. No sick fish.

So, I got some limestone from jordan. I gave most of it to my mom because she is a rock nerd. I scrubbed a small piece and put it in my 10 gallon. It bubbled, which probably mean my tank is on the acidic side? This tank houses some small sailfin mollies, a bnp and 3 blue shrimp. I am skeptical of why people are so against shells and rocks if regular water changes are done and fish and plants are healthy and happy.

Just fyi i rarely test my water so I don't know how high exactly is the pH or hardness. I just watch the tank and make sure all is good. What are your thoughts and experiences?
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post #2 of 6 (permalink) Old 10-04-2015, 01:49 AM
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Shells and limestone will slowly dissolve in the water, leaching out calcium (GH) and carbonate (KH). If you want calcium and carbonate leaching into the water, then this is a good thing. If you don't want these leaching into the water, then it's a not so good thing.

The parameters of the water will change with shells and limestone in the water. The change in water will increase with time, so if you neglect to conduct a water change in a reasonable time period, then the water in the tank can become significantly different to the water you use for water change, which may be an issue when you do get round to doing that water change.

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post #3 of 6 (permalink) Old 10-04-2015, 01:54 AM
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Seashells and crushed coral, cuttlebone, limestone all contain calcium carbonate as the predominant mineral of concern. Over time this dissolves in water and adds calcium and carbonates to your water. Both ions are utilized by plants and livestock to varying extent.
Calcium increases general hardness (GH)
Carbonate increases carbonate hardness (KH)

The mollies like harder water, don't they? Not sure about bristle nose plecos.
Blue shrimp- neocaridinas or tigers? Either way they will use the calcium for sure.

It's probably not an issue unless you don't do regular water changes. Or if you have livestock that prefers soft water.

I have soft water and deliberately add crushed coral to some tanks. I have a tank with local limestone. I just monitor and do regular water changes.

Gee, I hate repeating what someone else has said. But I guess that's what happens when you're typing and the page doesn't refresh before you submit reply.
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post #4 of 6 (permalink) Old 10-04-2015, 02:19 AM
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Fish Em,

the short answer to your question is: It is ok for the fish to live in tanks that have sea shells or lime stone in it. (No ill effects on fish or plants)

I suppose what you need to know is that both are composed of a mixture of Calcium and Magnesium carbonate (mainly CaCO3) which at pH 7 or higher is essentially insoluble in water.
However, in acidic water it will slowly dissolve because under acidic conditions the carbonate will react with water and form Calcium bicarbonate (CaHCO3) which is water soluble. The effect is that the acidity of the water will be neutralized and the concentration of calcium in the water will be increased! This is why shells or limestone (sometimes used in substrate) serve so well to buffer the pH of the aquarium water. Keep in mind though that because they slowly dissolve, the GH (General Hardness) of the water will slowly go up.

A problem can occur when you dose CO2 into your tank because the pH in your tank is lowered, which results in a much higher dissolution rate of your shells or limestone.

Hope this helps.
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post #5 of 6 (permalink) Old 10-04-2015, 09:09 PM Thread Starter
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Thanks for the technical info. I didn't think about the co2 problems. But it is all very interesting. Sometimes I feel like I am in chemistry class reading here.
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post #6 of 6 (permalink) Old 10-04-2015, 09:18 PM
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Mollies are hard water fish, and will thrive in harder water. If the shells etc are decomposing in the tank, then the water is too soft for them.
I would add minerals to the new water you are setting up for a water change so that it is harder, higher pH. Better suited to Mollies. Then there will not be a swing in water parameters, and the shells etc will last longer in the tank.

Bristle Nose Plecos thrive in a wide range of parameters. Harder water should not be a problem for them.

I know some shrimp are soft water critters, so you might do some research to see if your shrimp are compatible with Mollies' water preferences.
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