What caused this coral to crash?

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Dave Wolfenden offers some advice for a reader who's suddenly lost all his pulsing Xenia.

Q. I’ve kept pulsing Xenia in my 300 l/22 gal reef for around a year now. I was really pleased with their growth and they all looked incredible to me.

Unfortunately, I recently lost virtually all of the colonies within a few weeks. The water parameters seem fine: pH 7.8, nitrite 0; nitrate <5ppm; and I’ve been feeding them regularly with liquid invertebrate foods. What could be the problem?

Barry Collins, Hull

A. Xeniids are renowned for overtaking aquariums with rampant, weed-like growth, then seemingly 'melting' for no apparent reason. While it’s difficult to suggest a single answer there are several possibilities — and perhaps a combination of all of them could be held to blame.

Your water parameters seem acceptable, but let’s be more specific about what to aim for.

Xeniids appear to be sensitive to pH fluctuations and excessively low pH. Therefore, aim for a minimum pH of 8.0, with 8.2 preferable. Your current pH of 7.8 may be a bit low.

To ensure that such a pH is stable, alkalinity should be around 12 KH. There is anecdotal evidence that iodine may be necessary for the growth of soft corals such as Xenia, so it might be worth testing and maintaining at 0.06ppm, with supplements being available if they are deemed necessary.

Aggressive behaviour and allelopathic chemicals might also be to blame, so check for corals such as LPS being able to sting them with their sweeper tentacles.

Using Granular Activated Carbon and ensuring efficient skimming may help remove 'chemical warfare' toxins excreted by other cnidarians which could harm the soft corals.

The anatomy of xeniids suggests that, due to their lack of a functioning digestive tract, they probably don’t feed and their 'nutritional' requirements are met courtesy of absorption of nutrients directly from the water, plus the actions of their algal symbionts. Therefore feeding these corals is unnecessary and it might pay to review the tank’s feeding regime in view of this.

Dave Wolfenden

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