FAQ: Plants for fish

Some aquarists build so-called soil tanks with soil under a sand cap, but this takes experience. Plain garden soil or composts with added fertiliser can pollute the water, so beginners are safer with substrate made for aquariums.

The substrate usually does not need replacing, especially if plants are well rooted in it. Depleted nutrient soil can stay in place and be topped up with root tabs; a complete change is best saved for a major rescape.

Siphon only open areas and the surface of the substrate, without pushing deep between plant roots. Organic matter in the root zone is useful to plants, and heavy digging can damage roots and disturb a nutrient layer.

Many nutrient soils lower pH and carbonate hardness, so they suit soft-water plants, fish and shrimp. If you keep fish that like hard water, such as African cichlids, check the substrate’s description before buying.

Aquarium plant fertilisers come as liquids added to the water, root tabs or capsules pushed into the substrate, and nutrient-rich substrates. Liquids feed through the leaves, tabs through the roots, so the two are often combined.

Liquid fertiliser suits plants that feed from the water: stem plants, floating plants and epiphytes. Root tabs are for plants that feed through their roots, such as cryptocorynes, Amazon swords or vallisneria.

It depends on the type of fertiliser and the lighting: some are dosed once a week after a water change, others daily in small amounts. Always follow the label dose and start lower, especially if you keep shrimp.

Root tabs are usually replaced every few months, as the manufacturer directs. Push them deep into the substrate near the plant’s roots so they do not dissolve into the water and feed algae.

Macronutrients are nitrogen, phosphorus and potassium, which plants need most. Micronutrients such as iron, manganese or boron are needed in small amounts, and a shortage of either group slows growth or changes leaf colour.

Iron is essential for chlorophyll, so when it runs short young leaves turn yellow or pale. Most all-in-one fertilisers contain iron, and separate iron fertiliser is mostly used for red plants or a clear deficiency.

Fertilisers made for aquariums are safe for fish when used as the label directs. The danger lies in overdosing or using houseplant or garden fertiliser, which can contain ammonia, excess phosphate or substances harmful to fish.

Most aquarium fertilisers are safe for shrimp, but shrimp are sensitive to copper and sudden parameter changes. Choose fertilisers described as shrimp-safe and start with a lower dose than stated.

It is not advisable, because houseplant fertilisers often contain ammonium nitrogen, urea or other additives that harm fish in an aquarium and feed algae. Aquarium fertilisers are formulated with aquatic animals’ safety in mind.

Change a larger share of the water, stop fertilising for a few days and watch the fish’s behaviour and algae growth. If the fish gasp at the surface or act unusually, change more water and ask a vet or an experienced aquarist.

If there are only a few slow-growing plants such as anubias or Java fern, fish waste and an occasional small dose of liquid fertiliser are often enough. Too much fertiliser in a lightly planted tank mostly feeds algae.

Weekly fertilisers are usually dosed after the water change so you do not pour them out with the old water. Some aquarists dose in the morning, before the lights come on, when the plants start growing actively.

EI (estimative index) is a method where plants get nutrients in surplus so they never run short, and the excess is removed by a large weekly water change. It is mostly used in tanks with strong lighting and CO2.

No, many undemanding plants grow well without added CO2, getting it from fish respiration and the water surface. A CO2 system speeds up growth and is essential for many carpeting and red plants under strong lighting.

There are pressurised systems with a cylinder, regulator and diffuser, yeast or citric acid and baking soda generators, and so-called liquid carbon. Pressurised systems are the most precise and long-lasting; generators are simpler but dose less steadily.

It is a liquid additive dosed into the water as an extra carbon source for plants instead of CO2 gas. It can help in modestly lit tanks, but some plants such as vallisneria are sensitive to it, so stick to the label dose.