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I want to add some rocks and tree branches to my aquarium for decoration. If I put them in the oven and cook them, will they be safe for the fish? What temperature and length of time is sufficient?

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Ensuring that the rocks and wood that you put into your aquarium is definitely an important step to take before adding anything. I've seen many, many horror stories of people adding wood/rocks to aquariums and losing all the fish in them due to incorrect preparation. Please do not take half measures with this.

Do not heat/boil rocks!!

Rocks can contain air or water pockets which, when heated, can explode! This can obviously be extremely dangerous. Whilst you may get away with it, it's really not worth the risk.

Rocks

You'll need to select rocks that are appropriate for your tank. You need to remember that adding rocks and wood to your aquarium can and will affect your (currently stable) PH. Large swings in PH can be fatal for fish.

Firstly, you'll want to do as Deither suggests and put vinegar on them to see that they are safe to add. If it reacts, you'll probably want to find another rock. If you have some acid to hand you can use that to get a better idea of how appropriate this rock is.

If you rock passes the test, you'll need to clean it up. Scrub it initially to get the dirt off. It may look clean but it could still be harmful to your tank. Now clean the rock in a very weak bleach solution. Be very careful using bleach near an aquarium. This can kill your whole tank (Check out resources like this if you are concerned about it). Once you have cleaned the rocks and all traces of bleach have gone then you can add them to your tank. (Again, don't take any risks here - ensure you know how to use bleach on aquarium items safely before using it).

Wood

Wood is often smaller and can usually be boiled in some way. If you have something like bog wood which will give off tannins in your water (making the water yellowy-brown), you'll definitely want to boil or soak it to release the tannins if you don't want brown water. Ensure that you carefully boil your wood to get rid of possible bacteria. If you intend to put recently severed tree branches into a tank, consider that these will still have sap etc under the bark which could also cause you problems.

In all honesty, wood from a local pet shop is going to be much safer and the cost of it really isn't prohibitive, in most cases. This really holds true for rocks too. If you don't want to risk any issue, play it safe.

Additional Resources:

  • Definitely check out this video which explains the best practices for getting rocks from river bank to aquarium.
  • Do not boil/heat rocks (It's worth mentioning so often).
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Rocks
Not all types of rock are suitable for aquariums.
But it's easy to test: put some vinegar on them, if you see small bubbles appearing they are not suitable.
If that's the case, the rocks contain too much chalk, which can have a bad effect on the aquarium and fishes. It makes the water harder and will affect the pH levels.

Wood
Again, not all types of wood are suitable for keeping submerged for a longer amount of type. It will start to rot and pollute the water or it will keep floating.
Have a look here for some more info: wood suitable for aquariums
In my opinion, it's safer to purchase some wood from a pet store.

Prepartion
Don't cook the rocks and wood! Putting wood in the over could be dangerous!

In stead do this (even with rocks/wood purchased from a pet store):

  1. Rinse and scrub the wood and rocks using a brush and tap water (do not use any chemicals). This is to make sure any dirt is removed.

  2. Next, you can boil the rocks for a couple minutes. This will kill any bacteria.
    For smaller pieces of wood you can do it too, but if you do it too long it can damage the structure of the wood. You can also pour some boiling water over it a couple times (I did this with my big pieces of wood).

  3. The rocks are now safe to add to your fish tank.
    For the wood it's better to keep it submerged in a bucket with water for some times (perhaps a day or 2).
    This will allow the wood to soak up water so it will sink to the bottom once you add it to your aquarium.
    Also, some types of wood can colour the water, by keeping it in a bucket for some time, you won't get (as much) colored water in your fish tank.
    Tip: fill the bucket with warm water (or pour the boiling water over it in the bucket), this will make it go faster.

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