How can I stop this 'weed' taking over my reef tank?

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Xenia is beautiful but it can be invasive in the aquarium.

I have a real problem with pulsing Xenia in my tank. I constantly cut it off and ensure everything comes out but it just grows back.  It is now starting to affect my SPS corals and clam. Is there a way to get rid of it that won’t involve stripping the tank down? The only thing it won’t grow up against is my Palythoa colony which seems to burn it back. Help!
LUKE, PORTSMOUTH

Pulsing Xenia can look fabulous, but at the same time it can become invasive, attaining weed-like status in many aquariums. It can shade out other sessile invertebrates and outcompete them for space, but studies have shown that xeniids also emit toxic (‘allelopathic’) chemicals which can harm neighbouring corals. Unfortunately, controlling it can be difficult, although it’s possible to confine it to specific areas of the aquarium (at least to a certain extent) by keeping it on isolated rocks away from other invertebrates. This can help curtail its spread throughout the aquarium. Biological control is hit-or-miss at best, and chemical control can work, but it runs the risk of polluting the aquarium as the Xenia dies off, so physical control is the best approach.

As far as removing the existing Xenia, bear in mind that they’re very good at regenerating from even the tiniest fragment left behind. Therefore, simply cutting them off at the base, or even scrubbing them off the rocks can leave tissue behind, allowing them to regrow. 

Where possible and practical, look into completely removing the worst-affected rocks and trade these with other aquarists or your local fish shop (someone will actually want them!). 
Otherwise, you’ll need to temporarily remove the rocks and physically remove the entire colony before replacing them. This means either chiselling or snipping away at the rock on which the Xenia is growing with a suitable tool (avoid getting any oil in the water from the tools, and wash them straight after use). On rocks that you can’t lift out of the water, this will be trickier, but it can still be done. Xenia that’s growing on the glass can be simply scraped off with a credit card or metal blade, but do this whilst syphoning at the same time to remove the animal. 

A complementary approach is to try and limit nutrients. Xenia don’t actually feed (they don’t have a functional gut or mouthparts), but they can take up nutrients directly from the water, so elevated phosphate and nitrate could encourage their growth. Having said that, some folks with pristine aquarium water still find Xenia a problem, but this is certainly worth looking into all the same. Ensure mechanical and chemical filtration (activated carbon and phosphate-adsorbing media) are working effectively, and the protein skimmer is suitably adjusted to pull out maximum organics from the water. 
DAVE WOLFENDEN