Lava rock has a very convoluted surface which we initially thought would mean it must be doubled in surface area calculations. This turned out not to be true.
Lava rock is best used in aquariums in the range of ¼ to ½ inch in diameter. You can get small lava rock from the internet. Joey (the King of DIY aquariums) recommends just using a hammer to take large lava rock to half an inch.
Many folks on FaceBook use lava rock of the same size as it comes from the garden shop, typically two to three inches in diameter. This is far too big of a media to be an effective biomedia. It has a surface area of around 2 square foot per cubic foot. This is ridiculously low. These large size rocks must be broken up.
Lava rock is just pumice or perlite with a bigger pore size and thicker rock walls. It doesn’t float so it can be used in all submersed static media bed filter designs. Since it is a crushed rock product it does shed tiny microscopic particles which can destroy the impellers in an aquarium pump.
Testing of Lava Rock
When tested half-inch lava rock did not do well when it came to ammonia oxidizing capabilities. This was quite honestly very surprising. We did not expect lava rock to perform this poorly.
A test of ammonia oxidizing capability of various filter media was run. The first number, the “efficiency” is the average ammonia oxidizing that 15 cubic inches of media accomplished over a 90-day period. The second number is the “effective” surface area in square feet per cubic feet calculated from that test. The third number is the effective surface area in square feet per cubic feet calculated by simple mathematics. The correlation between the test results and the calculated surface area is very significant and means the testing was accurate.
| Media | “Efficiency” from two tests | “Effective” Surface Area ft2/ft3 | ft2/ft3 from Math |
|---|---|---|---|
| 30 PPI Foam | 17 | 340 | 400 |
| Pot Scrubbers | 14 | 280 | 80 |
| Static K1 Media | 13 | 260 | 200 |
| Aquarium Gravel | 6 | 120 | 120 |
| Blue Matala Pads | 5 | 100 | 120 |
| Eshopps Bio Balls | 5 | 100 | 60 |
| 1/4 to 1/2 inch lava rock | 3 | 60 | 60 |
| Matrix | 3 | 60 | 30 |
| BioHome Ultimate | 2 | 40 | 30 |
| Ceramic Rings | 2 | 40 | 40 |
The higher the numbers here the better the media. Lava rock did not do too well. This test can be found in this article:
7.1.3. Test of Filter Media
Lava rock is just a porous natural ceramic product. As such it shares all the shortcomings of porous ceramic media. A write up on these shortcomings can be found in this article:
7.2.11. Ceramic Filter Media
Lava Rock Does NOT Remove Nitrate
There are websites (aquaessentials.co.uk and Aquariumfish.net) which claim the following about lava rock in an aquarium:
“You only have to inspect the rock and you can see it is covered in tiny holes making it extremely porous allowing water to pass through and diffuse into the rock. So what does The Hidden Benefits of Lava Rock really mean? An anaerobic environment is created inside the rock as beneficial nitrifying bacteria consume all the oxygen in the water. Within this anaerobic environment inside the rock, denitrifying bacteria consume the nitrate and produce oxygen and nitrogen. We all know how nitrate in the aquarium is bad news for fish and shrimp so lava rock really is the most natural and best way of removing nitrate.”
I can’t sugar-coat it. This statement is pure and simple hogwash. Note that the term “lava rock” in this quote is linked to websites that sell lava rock. Click on the link, buy some lava rock and the website gets a commission. Don’t you just love the profit motive?
Again, let me emphasize, lava rock categorically does absolutely NO removal of nitrates to nitrogen gas. It is completely and totally impossible. We tested two products with similar “microporosity” as lava rock (BioHome and De*Nitrate) and found no denitrification took place over the span of one year. This test can be viewed at this link:
7.5. Test of Denitrifying Media
Startpage Aquariumscience
Source: Aquariumscience.org – David Bogert
