Basics : Substrate
There are pros & cons to every aspect of substrates, but a good overall choice
would be a well-rounded gravel of about 1/8" diameter (with some variation in
sizes), in natural colours (brown, grey, red & beige).
Details...
The topics I've covered are colour, shape,
plants, sand hazard, landscaping,
depth, mineral inertness, effect
on filters, costs and epoxy coated gravel.
COLOUR
Lighter colours reflect more light, making the aquarium brighter. This would seem
to be a good thing, except it's not natural and many fishes react poorly when
the amount of light reflected from the tank bottom is high. In nature, the lake/river
bottom is dark, and fish will naturally orient themselves vertically to the light
source. Light colours will also show detritus and algae more quickly. A very dark
colour requires stronger lights to achieve similar results with plants. Gravel
colour is sometimes chosen to compliment (or highlight) the colour of the fish
(ie: red goldfish on dark blue gravel), or plants (don't use green gravel). Solid
colours tends to show detritus more than a mix of colours What colour is finally
chosen sometimes depends on the room the tank is in (matching the wall or curtains).
Rainbow and Neon coloured gravel often chosen by kids tends to lose it's novelty
effect with time, and then changing gravel colours later can be quite a chore.
SHAPE
Bottom-feeders (ie: Corys, Sharks and Loaches) use their barbels to taste for food bits in the gravel. If the gravel edges are sharp, the barbels can be damaged. Cuts on barbels are susceptible to bacteria living in the gravel, which cause the barbels to become short nubs, instead of the long tendrils you would typically see, if the substrate was (for example) sand. Tanks with bottom-feeders should have smaller grain rounded stones or sand.
PLANTS
Some plants (Anubius and Ferns) do well in coarse 1/4" gravel, but most plants do better in a finer 1/8" or smaller gravel. Very fine gravel (fine sand) may compact too tightly for some plants to do well.
SAND HAZARD, POISONOUS RELEASES
Very little detritus settles into sand, but what does is trapped, and broken down by anaerobic bacteria. The bacteria creates small pockets of noxious gases (think about a rotten egg sandwich) which can be fatal to fish when allowed to accumulate and released into the water column. For this reason, hobbyists often use Malaysian Trumpet snails (MTS) with sand substrates. These nocturnal snails churn the sand releasing poisonous gases into the water to be harmlessly dissipated. Gravel will also collect detritus to harmful levels. Here the solution is to gravel vacuum the debris on a regular basis (sand cannot be vacuumed, only its surface). If you have driftwood held in slate bases, it is recommended that you use a gravel vacuum at the base when you need to shift or remove the wood. It is very common to have these gases accumulate under the slate used to hold the driftwood down. Symptoms include death (especially to the closest fish when you were moving the driftwood) and distress to the rest of the tank. Solution is to drop in an airstone ASAP and proceed with a massive water change.
LANDSCAPING
The coarser the gravel, the easier it is to create slopes. Sand tends to be self-leveling.
It is desirable to have the substrate angling upwards towards the back of the
tank. This makes gravel vacuuming along the front the most effective, and the
filter's pick-up power is typically better in the front where there is less obstacles
to the water flow.
DEPTH
Opinions vary, but generally you should use the minimum amount of gravel which will achieve your objective. Deep gravel accumulates more detritus and is harder to vacuum. Planted tanks with substrate rooters will need a few inches of gravel or else use pots. Substrate is not strictly needed. Bare-bottom tanks are often used for fry, breeding and quarantine tanks. Detritus is easily removed from a bare-bottom tank, and unsightly until it is. A typically set-up would start with only enough gravel to cover the glass along the front, elevating to about 3" along the back of the tank. The mid-area would have enough depth for mid-ground plants, driftwood and/or rockwork, and the deeper section would anchor larger structures and larger plants.
MINERAL INERTNESS
Gravel (and sand is just finely ground gravel, typically silica) may be of a mineral which leeches calcium back into the water. This is desirable to raise the pH, harden and buffer the water for fishes which prefer these conditions (African Rift lake fishes, goldfish, livebearers and some rainbowfishes to name a few). If the tank water is already hard & alkaline, then the effect will be minimal, adding some extra buffer for pH stability. In an acidic soft water environment, the leeching will be much more pronounced, and undesirable when keeping riverine fishes.
EFFECT ON FILTERS
All aggregates are abrasive, especially sand (hence sandpaper ;~). If it gets into the impeller well, it will score the plastic sides, and erode the plastic blades of the impeller. The finer the aggregate and the closer the filter's intake, the more likely this will be a problem. Normal gravel will be too heavy to get into a filter. Fish can contribute to this problem. A pair of Firemouths frustrated by their excavation attempts refilling with sand figured out that they could make the sand disappear by blowing it at the filter's intake.
Note of interest: any type of a substrate, (and ornaments, plants, rocks etc) will decrease the effectiveness of a filter's mechanical filtration, by making detritus harder to pick up. Bare-bottom tanks are very easy to keep clean.
COSTS
Substrate is typically $1 per lb (and a lb per gallon), so savings can be achieved by comparing costs from a quarry or home renovation supplier. Playsand is probably the cheapest substrate, but it compacts very tightly and has a few characteristics which can be undesirable. Coarse sand produces better results. A sandblasting shop might sell you some and they carry various grades. Generally, you should take some precautions with the substrate chosen, including checking for calcium leeching, and washing/rinsing with hot water.
EPOXY-COATED GRAVEL
Found in various colours, basically a lightweight smooth gravel available in various
colours Can usually be added directly into the water column with a minimal amount
of particulate matter. Seems good for bottom-feeders, plants, and gravel vacuuming.
Initially some problem with the epoxy recipe, this is rarely the case today. The
only drawbacks I've noticed is that it's somewhat artificial looking (all grains
are identical colour and nearly identical in size), and loaches vanish into it
(very easy to burrow into), but they probably don't see that as a drawback.
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