It’s tempting to use gravel as a grow medium for an aquaponics setup: it’s cheap, easy to maintain, and familiar. In some situations, like backyard dutch bucket systems, it’s well suited. If you’re using gravel, it’s important to ensure that it doesn’t contain anything that could alter the pH of your system. Pea gravel, for example, refers simply to the size of the stones, but the material could be virtually anything, including limestone. One very basic way of checking for potential problems is to fill a large plastic bucket with water (not tap water) and measure the pH. Add a good sized sample of your chosen gravel and let it sit for 24 hours then check the pH again. If the pH has changed, your gravel is probably the culprit and you should find another option. If you’ve confirmed it is pH neutral, you’ll want a size that won’t clog up your growbeds and has no sharp edges to injure you as you work. You’ll also need to take weight into consideration. Gravel can be quite heavy and if you’re looking at a deep grow bed, you may end up with components that are difficult to work with or subject to breakage.
There are some excellent alternatives to gravel that may be more suitable, depending on your particular needs. You may want to consider lava rocks, which have several advantages over gravel but can be really rough on your hands and the roots of your plants. Expanded shale can still be hard on your hands, but it’s not as sharp as lava rock. Expanded clay is actually a great option on nearly every point, although its price may give you pause. It also takes some time to get waterlogged, so it may float at first. For size, weight, durability, surface area and the fact that it’s completely pH neutral, expanded clay is really hard to beat.
