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If you own a reef tank you probably haven’t thought about buying sponges. And perhaps you shouldn’t, given their many problems. But then again, they do look lovely…

On Balance, there are probably more reasons that you don’t want to keep a sponge in a reef tank than reasons you do. But that’s not to say that sponges aren’t cool beasts. I mean, what other aquarium resident could you punch a few times, cut up with a knife, punch again, throw into a blender, spread the bits around a tank and expect to regrow everywhere? 

Sponges are born survivors, as long as they stay underwater. They’re one of the simplest life forms around; not as advanced as corals or anemones, but slightly more advanced than single-celled protozoans. They live in the phylum Porifera, and they’re so basic that they lack organs. It’s better to think of a sponge as a colony of cells with little in the way of advanced interaction. Each cell does its own thing, and somehow the whole biological machine keeps running. 

Sponges are incredibly diverse. There are around 20 freshwater species (all bland) but some 8,500 marine species in an array of shapes and sizes: tiny clumps barely as large as a lump of chewed gum to gigantic barrel-shaped bruisers 1.8m in diameter; tubes and balls; branched and encrusting; soft and hard; smooth sponges, spiky sponges, sponges that look like slime on a rock. Some live in shallow tidal regions, while others can be found at abyssal depths of over five miles, in perpetual dark. 

Amongst hobbyists they are notorious for dying, and this is often down to one error alone — lifting them out of water. Sponges are extremely porous (as anyone who has used a natural sponge in the bath will know) but they lack the ability to contract (they have no muscles, so to speak). When exposed to air, their pores fill with air, which they are unable to expel. Submerged back underwater the damage is done, the bubble sitting like a cancer and gradually killing the flesh around it. 

Getting it right

If you want a reef tank that’s highly varied, and you’re prepared to take some precautions, then sponges can be brilliant at introducing interesting hues. Their colours are there for the reason it’s so often wheeled out in nature — to advertise their toxicity. Only a few fish have evolved the ability to eat a sponge (namely large Pomacanthid angelfish species, with the dwarf species leaving them alone), so unless you’re housing a specialist feeder like a Moorish Idol, one of the sponge-munching filefishes, or a triggerfish (which would chew up rocks if they could) then you’re generally safe. Oh, and nudibranchs. Those charming little slugs love to munch on sponge.

Not all sponges thrive in dark conditions, spending their lives tucked away in the recesses of a tank. There are obligate photosynthetic species that require a space under the light. Haliclona sponges, for example, thrive under moderate lighting in the aquarium, and have a pastel hue that can rival many a coral. 

Purchasing and adding a sponge is relatively straightforward. Ensure it hasn’t been exposed to air (make that abundantly clear to whoever you’re buying from if it looks like there’s a risk they’re going to ‘lift’ your sponge), and use a drip method to acclimate it to your tank conditions, by placing it (with its transport water) into a suitable container and dripping in water from the tank via an airline with a valve on it. Remove some of the water as the container is close to filling and repeat a few times, then transfer the sponge (still submerged) into the main tank. 

A few considerations prior to purchase. Most sponges (by virtue of being the same cell throughout) should be uniform in colour. Avoid those with patchy shades, especially those with grey or white areas that indicate dying cells. If you’ve bought your sponge mail-order and it’s arrived with some grey or white, you may be able to carefully remove it with a scalpel, but it could be an indication that your purchase has met air. 

Read the rest of the feature in the April issue, available to read instantly on our digital edition HERE  or purchase the print edition HERE

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