Substrate

The most natural substrate is sand, but this is not ideal for use with an undergravel filter. If you have an undergravel filter, a shallow layer of sand (less than 2 to 3 cm) can be placed on top of regular coarse gravel. The sand will need to be separated from the gravel using a gravel tidy, a sheet of mesh which will keep the two layers mixing. The sand will be give burrowing fish like flounders and eels something to root about it, without interfering with the correct function of the gravel as part of the filtration system.

Plain beach sand can be used, but typically it is rich in lime and consequently hardens the water. For those brackish water fishes used to more marine conditions this isn’t a problem, but those fishes that come from the slightly brackish, more neutral conditions higher up the estuary, too high a pH and hardness can be undesirable. Moreover, few aquarium plants do well in lime-rich sands. Unless sold specifically for aquaria or horticulture beach sand is often rather dirty and so does need to be cleaned well before use.

Silica (or silver) sand is a better choice because it is chemically inert, and so won’t affect water quality or act as a buffer. But because it is inert it will not take up chemicals used in the aquarium, including medicines, and then leach them back out. Silica sand is rarely sold in aquarist stores, but is inexpensive and widely sold in garden centres and horticultural outlets where it is particularly marketed for use with cacti and certain other house plants. Mixed with fine (pea) gravel and laterite, silica sand forms an excellent substrate for most aquarium plants. Because this mixture packs down with time, it becomes slightly anoxic, which many plants appreciate because this condition favours the release of minerals like iron that they need to grow. However, this combination cannot be used with an undergravel filter, but an undergravel or undertank heater works well to circulate water within the substrate. Also, unless a there is good plant growth with an extensive network of roots throughout the substrate, fine sands can become dangerous anoxic, harbouring anaerobic bacteria capable of producing a gas, hydrogen sulphide, that is toxic to fish. In tanks with few plants or none at all, the sand should be stirred on a regular basis and any organic matter siphoned away during a water change. To be fair, burrowing fish should quickly root out any live worms or bits of food that get hidden in the sand (catfish and spiny eels are both very good in this regard) but it is best not to take any chances and clean the sand thoroughly at least once a month and be very sure not to let it become too deep or compacted in any part of the aquarium. The depth of sand in an unplanted tank need only be enough for the fish to burrow into comfortably, something like 2-6 cm being adequate for flounders and spiny eels.

Course calcareous sands, such as coral sand, make good undergravel filter beds, either in the tank or in an external canister filter and have been widely used by marine aquarists in this way. As a substrate they are quite light reflective, which tends to make fishes ‘wash out’ their colours. For this reason it is not ideal. As with beach sand, freshwater plant growth is not reliable in coral sand. Marine algae, like Caulerpa , do very well in it.

Standard non-calcareous gravels are the most widely used substrates. As a general rule, for plants the finer the better. Fishes kept alone need only enough to cover the bottom (1-2 cm). For big non-burrowing fishes, like Monodactylus and gar, a good effect can be gained by mixing a medium to coarse grade gravels with water worn pebbles and a few boulders. Such a substrate is easy to clean and will not trap pieces of food. It also allows plenty of swimming space and avoids sharp edges that the fish might scratch themselves on. Burrowing fishes should be given only smooth, fine gravel, or sand of some sort, as mentioned above.