FAQ: Tank Decor for fish

Not all: choose decorations made for aquariums from water-resistant materials. Some rocks, shells or corals can raise water hardness and pH, and painted or metal objects can leach harmful substances, so read the product description.

Many fish hide among rocks, roots and plants in the wild, so in an aquarium hides reduce stress and aggression. They matter most for territorial, nocturnal and bottom-dwelling fish, and for moulting shrimp.

Gravel is easier to siphon and lets water through well, while sand suits fish that sift and dig in it, such as corydoras or loaches. The choice depends on your fish, your plants and the look you want.

Gravel of 2–5 mm usually suits an aquarium: it does not stagnate, is easy to clean and works for most plants. Gravel that is too coarse traps food scraps in the gaps, and for bottom fish with sensitive barbels a fine, rounded substrate is better.

Work out the amount by multiplying the base length by the width and the layer depth you want, usually 4–7 cm. For example, a 60 × 30 cm base with a 5 cm layer needs about 9 litres of substrate; in kilograms it depends on the density given on the pack.

Coloured gravel made for aquariums has a water-resistant coating and is safe. Avoid painted substrate of unclear origin, because the coating can flake off, and bright colours stress some fish and wash out their natural colouring.

A dark substrate looks more natural to many fish and brings out their colours, as fish do not fade against a dark background. A light substrate brightens the tank, but some fish become paler and more nervous on it.

Put the new gravel in a bucket in batches and rinse it under running water, stirring by hand, until the water runs clear. Do not use soap or detergent, and do not rinse nutrient soil unless the manufacturer says to.

Cloudiness is usually caused by fine particles or dust left in unrinsed substrate. Next time rinse it well and pour water onto a plate or bag so the stream does not stir the bottom; usually the cloudiness settles within a day or a few with the filter running.

Clean gravel with a gravel vacuum during water changes: push the tube into the substrate so debris is sucked out while the gravel stays put. Cleaning part of the bottom every week or two is usually enough, rather than all of it at once.

Gravel does not need replacing: beneficial bacteria live in it, so regular siphoning is enough. Replace it only to change the tank’s look or a worn coating, and do it in stages so you do not upset the biological balance.

Change the substrate in stages over several weeks: each time remove some old substrate with a siphon or cup and add rinsed new substrate. That keeps some beneficial bacteria and is less stressful for fish than redoing everything at once.

You can, but it is risky: gravel collected outdoors can contain lime, pollutants or parasites. It must be rinsed thoroughly, tested with vinegar for lime and boiled or dried, and for beginners substrate made for aquariums is safer.

Black patches, bubbles and a rotten-egg smell show that the deep substrate lacks oxygen and hydrogen sulphide is forming. It mostly happens in a deep, very fine sand bed, so stir it gently and regularly, but not all at once.

A bare bottom is easy to clean, so it is used in quarantine, fry and some goldfish tanks. Without substrate, though, there is less room for beneficial bacteria, nowhere for plants to grow, and reflections from the glass stress some fish.

Sand made for aquariums or washed quartz sand with rounded grains, usually 0.5–1.5 mm, suits an aquarium. Builders’ sand is often dirty, limy or sharp-grained, so it is better avoided.

Some aquarists use clean quartz pool filter sand because it is inert, but it must be rinsed thoroughly. Play sand often contains impurities or dye, so check the composition before use and make sure it contains no lime.

Clean sand with a siphon held just above the surface so only the debris is sucked up and the sand stays behind. Sand is heavier than debris, so the dirt gathers on top and is easy to remove.

Corydoras, loaches, some cichlids such as eartheaters, and other fish that sift sand through their gills like a sand bottom. Sharp or coarse grains can damage their barbels and mouths.

Fine sand can get sucked into the filter if the intake is close to the bottom or the fish stir it up. Raise the intake, fit a sponge over it and choose slightly coarser, heavier sand.