Why are my plants turning black?

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One of our readers asks for advice on how to stop their plants from turning black in their tank...

Q) Please could you advise me on what I can do to stop the leaves of my plants going black? The fish are okay, and I do a water change every week of 25% using water conditioner and plant food. The light is on for eight hours a day. My tank is a Juwel Vision 180. The temperature is 25°C, 7.6pH all water parameters test fine.

EDDIE KITTRIDGE, VIA EMAIL

A) Neale says: This is Rhodophyta algae, and there are several types that infest aquaria, including hair algae, brush algae, black beard algae, and black spot algae. These algae all tend to grow best in tanks where the growth of higher plants is indifferent. This is often explained by supposing that 'something' has to use up the nutrients in the water, and if plants aren't growing quickly, algae will do the job instead.

Often the first step to addressing algae problems is to determine how quickly your aquarium plants are growing. If you're removing a handful of excess leaves every week or two, that indicates healthy plant growth. If, on the other hand, your plants are not really doing much, and you can't remember the last time they needed pruning, then that can be informative.

Of course, some plants grow slowly anyway, notably things like Anubias and Java ferns and almost without exception these plants become 'algae magnets' if placed under direct light. They are much happier in tanks stocked with floating plants or some other overhead vegetation that reduces the amount of light that reaches them while also growing fast enough to remove nitrate and phosphate from the water.

Alternatively, choose undemanding but fast-growing stem and rosette plants. This a good way to control algae, and once these are tall enough to shade the slower growing plants, you should find fewer black patches on their leaves. The Rio 180 is an excellent aquarium, and the LED lighting built into its hood is pretty good. While you may find the more demanding plants struggle without additional lighting or carbon dioxide fertilisation, but the more adaptable species should do fine.

Another trick recommended by many fishkeepers is to split the lighting period into two 4-hour sections with a 1-2 hour 'siesta' between them. While this may not cure your algae problem by itself, it might help somewhat.

6 great plants for algae control

Water sprite or Indian fern, Ceratopteris thalictroides: This fast-growing bushy plant is a good pick for algae control, and can be grown either planted in the substrate, or as a floating plant. The rooted form has slender, feathery leaves that, if snapped off, give rise to the floating form eventually. Reaches over 30cm in height when grown rooted.

 

Amazon frogbit, Limnobium laevigatum: The leaves of this attractive floating plant help to prevent algae by shading areas of the tank and using up the nutrients in the water that algae feed on. The roots trail down into the water and provide cover for fish.

 

Vallisneria spiralis: Vallis is able to use bicarbonate ions as a carbon source, so provided your water is at least moderately hard, you will likely find these species do rather well. Only bury the bottom 90% of the roots and ensure the white crown part of the plant at the top of the roots is well above the substrate. Grows to 50cm or more.

 

Hygrophila polysperma: An undemanding plant that does well in most aquariums. Reaches up to 40cm in height.

 

Limnophila sessiflora: This attractive, fast-growing bushy plant grows to 40cm and is easier than the more light-demanding Cabomba.

 

Red Ludwigia, Ludwigia palustris: A beautiful plant that turns red under good lighting. Much easier to grow than many other red plants, it reaches 30cm or more in height.