Star Polyps and alkalinity

[oranje]

In one of my tanks, I unfortunately let the alkalinity drop to around 4dKH... bad reef keeper. But, the only reason I noticed this is because one of my leather corals was looking fairly sickly. All the other corals in this tank, including mushrooms, branching frogspawn, button polyps, and star polyps, were doing fine. So, I raised the alkalinity over a few days to 8dKH.

The leather coral is now just fine, as are virtually all of the other corals. However, one of the varieties of star polyps will not come out with the alkalinity this high!

The coloring of these is virtually NEON yellow/green... as opposed to the brown and green patches elsewhere. Upon letting the dKH drop down to 5dKH(too low!) again, they came out. Bringing the alkalinity up again caused them to recede! The pH has remained stable at about 8.3, calcium at 400-450ppm, no ammonia, nitrites, low nitrates. And all of the other corals are perky and fine.

Are some corals intolerant of "normal" alkalinity? I realize that sea water naturally has lower alkalinity than what aquariums should be at, but should I be keeping my alkalinity in the "normal" range if it prevents one of my corals from coming out?

 

[Len]

I've had similiar experiences with GSP. However, what I've concluded is that they respond negatively due to environmental changes, not because of high or low alkanity. In other words, they're very sensitive to a changes in a system's physical and chemical parameters. It sometimes takes up to 2 weeks after severe changes (e.g. post water change) for my GSP to reopen again.

 

[Anonymous]

i've noticed in my systems that soft corals tend to do better (greater polyp extension) in a lower alk. environment such as 2.2meq- 1.8meq range. if kept at more standard aquarium levels 3.0- 4.5, the corals shrink in size rather dramatically. i don't know if polyp extension is an accurate means to judging a corals preferances but thats all i know to gauge by.