Salty Q&A: Acropora Bleaching Fast!

Question

I just introduced an Acropora to my 65-gallon reef tank, and it’s already starting to bleach on me. I’d say about one-quarter to one-third of the colony has already bleached, and it’s been in the tank less than a week. This is my first acro. Apart from it, I have several different varieties of soft corals and zoa colonies. The fish include 6 blue-green chromis, 1 royal gramma, 3 polka-dot cardinalfish, and 1 lawnmower blenny. All my water parameters are fine, and my lighting (high-output T5s) and water movement are both good. I placed the Acropora high in the tank close to the lights, so I know insufficient light isn’t the problem. What do you think is causing the bleaching? All my other invertebrates are doing great.” – Submitted by Maris

Answer

Thanks for your question, Maris. Well, any number of things could be going on here, and the information you’ve provided hints at several possibilities. First, the fact that your tank is stocked predominantly with soft corals and zoas suggests to me that your tank might be a “chemical soup” of allelopathic compounds. In other words, all those softies and zoas are likely waging “chemical warfare” on the new acro, which could cause the rapid bleaching you’re observing.

I also wouldn’t rule out an issue with your water parameters just yet. You state that all of them are “fine,” but absent any actual test results, that doesn’t tell me much. Generally speaking, Acropora corals are far more demanding than most softies when it comes to the levels of nitrate, phosphate, and other dissolved pollutants they’ll tolerate. Likewise, acros are generally much less tolerant of fluctuations in parameters such as water temperature, pH, salinity, and alkalinity. So what’s good enough for soft corals may or may not be good enough for finicky stony corals.

Similar logic applies to lighting and current. Acros demand high-intensity lighting and robust water movement, whereas many soft corals tend to be a bit more forgiving in both regards. You can’t necessarily assume these elements are sufficient for keeping acros in your tank just because soft corals are thriving there.

The last possibility I’ll cover here is that your new acro might be suffering from photo shock. You note that it’s positioned high in the tank and close to the lights, but you don’t mention whether you took any steps to acclimate the specimen to your lighting. I bring this up because there’s a good chance that the light it’s currently receiving is significantly different from what it had been exposed to in the days and weeks prior, which could shock it to the point of bleaching—even if the light level is technically appropriate for Acropora spp.

Again, these are just some of the possible explanations for your coral’s bleaching. If any other salties out there would like to weigh in to help us troubleshoot this problem and further the conversation, please do so in the comment section below.