7.2.9. Lava Rock

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Author : David Bogert

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In the short version: testing has shown lava rock is NOT a good media to put in any filter.

The data from the master table for lava rock was:

surface area calculations for lava rock
surface area calculations for lava rock

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 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 in 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 larges 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.

Yellow OB Peacock
Yellow OB Peacock

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“Effective” surface area m²/m³m²/m³ from math
Fluidized K1 medianot tested600na1980na
30 PPI foam1734040011221320
Pot scrubbers1428080924264
Static K1 media13260200858660
20 PPI foamnot tested220180726594
Aquarium gravel6120120396396
Blue Matala pads5100120330396
Eshoppe bioballs510060330198
¼ to ½ inch lava rocks36060198198
Biohome ultimate2403013299
Ceramic rings24040132132
* average ammonia oxidizing that 15 cubic inches (245,81 cm³) of media accomplished over a 90-day period

The higher the numbers here the better the media. Lava rock did not do too well.

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 website that sells lava rock. Click on the link, buy some lava rock and the website gets a commission. Don’t you just love the profit motive?

Lava Rock that Supposedly Denitrifies
Lava Rock that Supposedly Denitrifies

Again, let me emphasize, lava rock categorically does absolutely NO removal of nitrates to nitrogen gas. It is completely and totally impossible.