FAQ: Tank Decor for fish

You can, but over time the sand settles beneath the gravel, so the layers blend. For clear zones, separate them with rocks or plastic strips, for example sand in the foreground and gravel under the plants.

Algae, fish waste and leftover food show more on white sand, so it looks dirty sooner than a dark substrate. It needs siphoning more often, and the brown diatom film of a new tank is especially visible on it.

Black substrate suits most fish and brings out their colours, though dust and lime deposits show more on it. Choose black substrate made for aquariums rather than painted material, so no coating flakes off.

Quartz sand and most gravel made for aquariums are inert and do not change the water. Coral sand, aragonite and some limy gravels raise hardness and pH, while nutrient soils often lower it.

Coral sand and aragonite raise hardness and pH, so they suit substrate for African lake cichlids, some livebearers or brackish tanks. They do not suit soft-water fish such as tetras or discus.

Inert rocks that do not change the water are safe for an aquarium: granite, quartz, slate, basalt, lava rock and most rocks sold for aquariums. Limestone, marble and some others raise hardness and pH.

Drip vinegar or citric acid solution onto a clean rock: if it fizzes, the rock contains lime and will raise hardness and pH. Also check for metal veins, rust stains and sharp edges.

Yes, if the rock passes the vinegar test, has no metal veins and is scrubbed thoroughly and rinsed with hot water. Avoid rocks from polluted places, roadsides and industrial areas, as they may carry contaminants.

Rocks can be scalded with hot water, but boiling or baking them is not advisable, because trapped water or air pockets can make a rock crack. Scrubbing with a brush and rinsing well is usually enough.

Dragon stone (ohko) is considered inert and barely affects water hardness, so it suits most aquariums. Its pores hold clay and dirt, so it needs a good scrub and rinse before use.

Seiryu stone contains lime, so it gradually raises water hardness and pH. That is fine for hard-water fish, but in a shrimp or soft-water tank use it with care and monitor water parameters.

Lava rock is light, porous and inert, so it suits an aquarium and gives beneficial bacteria and mosses a good surface to settle on. Some lava rocks have sharp edges, so for bottom fish it is worth smoothing them down.

Slate is inert and splits easily into flat pieces, so it is handy for building caves, terraces and spawning sites for cichlids. Before use, check it for metal inclusions and smooth the sharp edges.

Limestone, especially porous Texas holey rock, raises hardness and pH, so it suits African lake cichlids and other fish from hard, alkaline water. Do not use it in tanks for soft-water fish or shrimp.

Marble is a limy rock, so it gradually raises water hardness and pH. It can suit hard-water fish, but for most tropical tanks inert rocks are the better choice.

Artificial rocks made of resin or ceramic for aquariums are light, inert and often hollow, so they suit hides and do not weigh down the base. Choose ones with a smooth surface and no flaking paint.

Place large rocks directly on the bottom glass or a protective base, and only then pour substrate around them, so fish cannot dig them out. Test stacks with a push, and glue rocks together with aquarium-safe adhesive if needed.

They can, if a rock is dropped or its weight rests on a small point. Lay a base of foam, an aquarium mat or plastic egg-crate on the bottom, lower rocks in carefully and never drop them into the water.

The amount of rock is limited by the weight the tank and stand can bear and by the open space fish need for swimming. Rocks displace water, so a rock-heavy tank holds less water for fish; factor that in when deciding stocking.

Yes, rocks with rust stains, shiny metallic veins or iron inclusions can leach metals that harm fish and especially shrimp. Do not put such rocks in an aquarium.