Will my pleco eat my shrimp?

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A reader worries his shrimps will be safe sharing a tank with a Pleco. PFK expert Bob Mehan advises.

My pleco, called Lochy (after the Loch Ness monster), is just over nine years old and about 32cm long. He’s a good gardener, and shares his 250 l tank with some small fish. I’d like to add a few shrimps to this tank for a bit of variety. There are plenty of plants and caves but will the shrimp be safe? 

KENNETH SHARPLES, VIA EMAIL

Bob replies: You don’t say which species of pleco you have but it seems likely that it could be the Common pleco, Pterygoplichthys pardalis, often erroneously sold as an algae eater. With an adult size in excess of 35cm, these fish seldom live up to their full potential, often crammed into undersized aquaria, and usually end up causing problems themselves due to the huge amount of waste they produce. However, it sounds as though ‘Lochy’ might be an exception given his size, and it suggests that you’re providing a diet beyond what he can scrape from the glass. 

Shrimp can make fascinating aquarium subjects and are extremely industrious residents — in fact, they’re far better at algae control than plecos. You don’t mention which of the many species available you’re considering, but the more exotic, colourful species such as Bee, or Crystal red shrimp, Caridina cantonensis, would be a poor choice due to a greater sensitivity to water chemistry and their small size. The best option would probably be Amano shrimp, Caridina multidentata, which grow to a reasonably large size and so are unlikely to be immediately hoovered up by Lochy, and are surprisingly fast swimmers which should be able to evade any unwanted attention. However, all shrimp are vulnerable to predation from otherwise peaceful tankmates when they moult their carapace to grow, leaving them soft and totally defenceless for around 24 hours. At this stage Lochy may be tempted, especially as the shrimp are less mobile. To prevent this, make sure there are hiding places in the form of caves which are too small for Lochy to get into, where the shrimp can seek refuge while their new carapace hardens.