Well I did the research I said I would and loach is definitely right and I was initially wrong... and lucky in the past. ROCKS CAN EXPLODE WHEN HEATED. Couldn't find too many reliable sources about this on aquaria sights so I had to turn to other sources. The Boy Scouts. They use heated rocks all the time to cook with, so they can be heated safely. But you have to be VERY careful about it.
The other sources I found were various scientific articles about rocks.
Below follows instructions about how to prepare rocks for the aquarium excerpted from my website. It was put together after completing the research I said I would for this thread.
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Any of the rocks that we find in rivers, lakes, ponds or other natural landscapes can come with other problems besides the ones we discussed above. These rocks can come loaded with parasites, bacteria, fungus or other organisms that can infest our tanks with unwanted visitors. In addition to these organic pests, rocks can be contaminated by pesticides, herbicides or other toxins that might kill our fish outright. So it is imperative that we prepare them properly. This is one of the reasons that I like boiling any rocks I find. Boiling will help to remove most of the toxins that may be in them. I would still avoid using any rocks that you know to be contaminated with toxins.
WARNING: BOILING OR HEATING ROCKS CAN POTENTIALLY BE VERY DANGEROUS. HEATED ROCKS CAN EXPLODE IF THEY ARE NOT HEATED OR PREPARED PROPERLY. CARE MUST BE EXERCISED IF YOU ATTEMPT TO DO THIS. IF YOU FOLLOW THE DIRECTIONS BELOW, YOU SHOULD BE SAFE. BUT, YOU MUST FOLLOW THEM EXACTLY!! IF THE ROCKS AREN'T DRIED OUT BEFORE USE, IT WON'T MATTER HOW SLOWLY THAT YOU HEAT THEM. THE WATER TRAPPED INSIDE WILL STILL TURN TO STEAM AND CAUSE THE ROCK TO EXPLODE!
There are 2 main ways to prepare rocks for the aquarium, the chemical and thermal methods. Both methods require the rock to be prepared in the same way before the sterilization process can begin.
Scrub the rock thoroughly with a brush bought specifically for this purpose. Dont use a brush that has been used for anything else. You could end up scrubbing chemicals INTO the rock if you do. A potato brush is a good kind of brush to buy for this. Dont use any kind of soap or chemical to clean the surface with as they can remain in the rock. Use only clean, clear water. Soft water is better to use than hard, but it doesnt really matter too much what you use.
At this point you need to decide which method you are going to use to sterilize your rock.
The chemical method is faster, but doesnt penetrate beyond the outer layers of the rock. It comes with some potential risks to your fish since some of the bleach used may remain on the rock. This can potentially harm your fish if the bleach is not completely rinsed away before adding the rock to your tank.
The thermal method takes longer and will do a better job of sterilizing the rock for your tank, as the heat will penetrate all the way through the rock. But it comes with some potential risks for you. If not properly prepared, the rock can explode. This may injure you and will certainly damage anything you are heating it on if it happens.
Now its time to sterilize our rocks!
Chemical Sterilization
Take a bucket and fill it with enough cool water to cover the rocks, but not so much it will overflow when you add the rock. Do not use warm water, it will cause the bleach to evaporate and defeat the purpose of soaking it in the first place.
Add 1-2 capfuls of bleach to the water in the bucket. Add a little more if it is much larger than 3 gallons.
Soak the rock for about a day in this solution
Rinse the rock THOROUGHLY. Scrubbing it with water flowing over it is a good idea here. The goal is to clean the rock until you can barely detect the odor of bleach. Its better if you cant smell any, but the sensitivity of your nose may have diminished from the odor of all the bleach water.
Soak the rock in a bucket again with at least 3 times the amount of water conditioner you would normally add to similar volume of water for about 3 hours.
Add the rock to your aquarium!
Thermal Sterilization
Once the rock has been cleaned and scrubbed place it somewhere warm and dry for at least a week. Placing a thermal pad under it can speed this process. This will allow the rock to dry out as much as possible. We are trying to remove as much of the water from the rock as possible to reduce the possibility of a steam explosion occurring during the boiling process.
Now that your rock has dried out, set-up a large stock pot with enough water to cover the rock and place it on a heat source. You can do this on the stove or outside with a propane stove. Boiling rocks can sometimes release unpleasant odors and some people prefer to do this outside.
Place the rock into the water BEFORE you begin applying any heat to it. It is VERY important that the rocks heats up slowly along with the water. This allows the heat to remain even throughout the rock.
Boil the rock for at least an hour adding water as needed.
After the hour has passed, let the rock and water cool BEFORE you remove them.
Rinse the rocks off a final time in tap water with a light brushing.
Place the rocks in your aquarium!
Thermal Method Variation
This is a faster method of preparing rocks, but potentially dangerous. It is offered as an option if you dont feel comfortable with the other two. You will need a good thermometer so you can monitor this process closely. You can heat the rocks and water 180°F for a 2-3 hours. Make sure that the temperature does not rise above 180. That is hot enough to kill most disease organisms. Continue to add water as needed. You need to stay nearby to ensure that the water DOES NOT BOIL!
Wait until the rock and water has completely cooled before removing them.
Rinse the rock off under the tap with a light brushing.
Place the rock into your tank.
This thermal method may not as sure as the other, but is offered as an option to those who are uncomfortable with the idea of boiling or using chemicals. The author has yet to try this himself, but has heard of it being done. I can offer no guarantees of its safety or viability, but the theory is sound and should work.
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I was asked to direct link to this page, so you can follow
this link to it.