What is this strange thing in my tank?


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Slime molds feed on decaying organic matter, so their presence can indicate a problem. Image by Luke Glover.

I recently set up a planted Fluval Edge 23 l/5 gal tank, after a few weeks and some new occupants later I noticed this ‘creature’, which I’ve since discovered could be an aquatic slime mold — but that’s where the info ends unless you understand scientific talk.

It seems to move around the tank, along the wood, occasionally disappearing into the substrate. Whilst I never see movement, if left for a period of time it will have relocated to another part of the tank.

For now, I’ve manually removed it with a syphon and toothbrush. But where did it come from and how do I stop it coming back?

Slime molds — those that are big enough to see, at least — are uncommon in aquaria, though small colonies could easily go unnoticed, so it’s hard to say whether or not they’re a normal part of freshwater set-ups.

What we can say is that they’re a bit like fungi in habits, consuming organic matter such as decaying plant material. That being the case, they’re not likely to threaten your fish directly. but just like fungi, they are a sign that’s something amiss with the aquarium. If you can see slime molds or tufts of fungi, there’s something in the tank decaying that they’re digesting. In doing so they’re producing waste products that the filter has to deal with, ultimately raising nitrate levels between water changes.

The fact that your tank is very small highlights some possibilities. A deep substrate can be very good at accumulating debris that the slime mold could be feeding on. It’s also very easy to overstock a small aquarium or overfeed the tank residents, and the faeces and uneaten food will provide food for your slime mold.

The one time I’ve dealt with slime molds in an aquarium was after adding fresh wood as decoration, as opposed to cured bogwood. Again, organic material in the wood provided food for the slime molds.

A lack of water circulation can also prevent organic debris from ending up in the filter where it can be processed by the bacteria, providing more food for the slime mold.