Are these materials safe to use?

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A reader asks advice on the best and safest way to create depth in his aquascape.

I’m a newcomer to fishkeeping and in the process of setting up my first freshwater tropical aquarium.

In an effort to create depth within the aquascape, I’m thinking of using a rigid plastic grid below the substrate. This would hold soil and gravel in place and also provide a strong base on which to place rock. These grids can be cut to size and shape and are normally used in garden landscaping and car parks to hold gravel in place. I believe they are made from recycled plastics. Could they be used safely?

I also intend to use some old rocks taken from my garden rockery. They are similar in appearance to Frodo or Seiryu. What is the best way to sterilise hardscaping material before placing it into an aquarium?

RICHARD PRIESTMAN, VIA EMAIL

Neale replies: It’s pretty common to use plastic ‘egg crate’ at the bottom of tanks to prevent rocks slipping and cracking the glass. The problem is that many fish, such as cichlids and large catfish, will dig around or underneath rocks, and without the egg crate there, sometimes the rocks can tumble or roll onto the glass. Nothing ruins the hobby more than having to deal with a leaky aquarium at 8pm on a Sunday night! The egg crate is impossible for the fish to move, so even if it’s exposed by their excavatory endeavours, the rocks will still be securely held above the glass.

This approach works well, and since it’s easy to hide the egg crate under the gravel or sand, it’s essentially invisible so far as aquascaping goes. Since egg crate is produced for handling food, it should be safe in an aquarium and shouldn’t leach out any chemicals. 

That said, I’m not so sure about the sorts of plastic grids sold for use on grass verges and gravel pathways.
You’d really need to consult with the manufacturer of the mesh in question. Most plastics are inert, but not all of them, and recycled plastics don’t necessarily have the purity we need to be safe.
High-density polyethylene (HDPE), for example, is used for all sorts of food containers as well as pond liners. We could rightly expect that to be perfectly safe in the aquarium as well. But with a recycled plastic, you can’t be sure it’s entirely made from a single type of aquarium- or pond-safe plastic.

So far as rocks go, the main thing is to ensure they’re free of metallic seams and that they’re pH neutral. Metals can be toxic to fish (particularly copper), and to test for lime, add a little acid — if you see fizzing, such rocks are likely to raise the pH of the tank. 

Sterilising rocks isn’t necessary. Unless they’re porous rocks, any debris on the outside, such as cobwebs or dust, is easily scrubbed off under a hot tap, and once cleaned, there’s really no risk of them introducing harmful pathogens into the tank. Porous rocks can be a bit trickier, and a good soak for a few days is well worth doing. Any little critters in such rocks will find their way out, and after that, a good clean under a tap should do the rest. But, if in any doubt, always stick with rocks intended for aquarium use — your local retailer should be able to help you there.