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Aquarium Rocks: Safe and Unsafe Rocks to Put in Fish Tanks

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Zach's writing ranges from matters of gardening, cooking, aquariums, and fish to more niche topics like coin collecting.

The large rock featured in this 10-gallon tank weighed more than the other rocks and the sand put together.

The large rock featured in this 10-gallon tank weighed more than the other rocks and the sand put together.

Safe and Unsafe Rocks for Aquariums

Natural-themed aquariums have the aquatics world splashing about with some of the most unique biotope designs I've ever seen. Sadly, I haven't seen enough rocks. No, I'm not talking about pebbles or golfball-sized rocks; I'm talking about boulders! Well, aquarium-sized boulders, that is.

By now, you've probably got me pegged as a mad man, as combining rocks and glass usually doesn't end up well, but give me a chance. Surprisingly, fish tanks are very strong these days, and with the proper precautions, just about any size rock can safely fit into an aquarium. In order to use rocks in aquariums, you'll first want to know what types of rocks are compatible with aquatic life, and then prepare your tank for the added weight.

How to Choose Rocks Compatible With Aquatic Life

In nature, aquatic life lives around all types of rocks. The aquarium, on the other hand, is a different story. Since an aquarium is a micro-ecosystem that is not able to dilute harmful minerals/toxins like nature, only certain rocks should be used. So if you're going to throw in a rock from your backyard, make sure you've got a safe one! Below is a list of rock types that are and aren't safe for use in the aquarium.

List of Rocks

Safe Rocks for AquariumsUnsafe Rocks for Aquariums


Lava Rock





How Can I Make Sure a Rock Is Safe?

A good way to test for safe rocks is to drop a couple of drops of pH Down on the rock in question. If it fizzes, it's unsafe and shouldn't be used.

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How to Prepare Your Tank for Heavy Rocks

The ideal aquarium to have for using large rocks is a flat-bottom acrylic tank, but glass aquariums can also be used. Here are the steps to ensure aquarium integrity while using large rocks:

  1. Get a stand that offers full bottom support and level it. Make sure that your stand has a flat surface to support the entire bottom of the tank. Metal or other stands that only offer support around the edges of the aquarium shouldn't be used. Once you have your stand in position, make sure that it is level. Keeping things level will ensure that there is no unequal weight distribution on the frame of your aquarium.
  2. For glass aquariums, attach Styrofoam under the tank for support. The next step is to ready your aquarium. Owners of acrylic tanks can skip this step. For glass aquarium owners, go to the hobby store and pick out enough Styrofoam sheets to cover the bottom of your tank. You'll want to have enough so that it basically fills in the raised portion between the glass and where the stand will be. I like to use tape to gently attach the Styrofoam to the bottom of the tank. This step is important because the Styrofoam will add support to the bottom pane of glass.
  3. Add a buffer between the rock and the glass. Don't put your rocks directly onto the glass! What you'll want to do is put sand or fine gravel down first and then place your large rocks on top of the layer. This prevents the large rock from scratching or cracking the tank due to a sharp edge.
  4. Fill the tank. Once the rock is securely in position, you can begin filling the tank with water.

Make Sure Your Rock Fits Your Tank

Following these steps will help you to have a unique tank that you'll be proud to show off. But remember to use your brain! Don't try to put a massive rock in an aquarium that is just too small. Reason is the food of the wise man. Good luck with your aquatic adventures.


Andrew from Rep Boston MA on August 24, 2011:

Nice useful guide on rocks to put with your little fishes (or big fishes in some cases). I always liked putting those glass rocks almost like marbles on the bottom of the tank and have a large plastic rock in the middle. Theres lots of choices and its good that people know the dangerous rocks to put into the tank.


Nspeel from Myrtle Beach on August 24, 2011:

I love aquascaping with all my aquariums it is just an amazing time to re decorate your coral reef. I only do it every several years but it is a blast.

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