I have kept the torch coral since setting up the tank and the leather coral since its early days, so I have grown especially attached to them. Compared with my other, relatively still corals, their gently waving polyps are what make them so appealing.
It was hard to watch when their polyps pulled all the way in and they shrank. I kept coming up with theories and trying things, and it really wore me out. In the end, though, it seems the biggest problem was the temperature.
After buying an OKE-6428HC cooling/heater temperature controller and setting it to 25°C, the torch coral is definitely looking better.
The mouth is still visible in the middle, but the outer polyps are growing well and it looks fuller. The clownfish looks happy too.
It was probably a buildup of several factors: a single head splitting into two, high temperature, insufficient nutrients, and so on. But in my tank, temperature felt like the biggest factor.
I tried all sorts of things—phosphate dosing, adding activated carbon, and more—but it turned out the glass thermometer was the issue…
If your corals have been looking worse and you have only been using a glass thermometer, it might be worth checking its calibration.
I was looking at the number on the sensor and something felt off. The glass thermometer stuck on right next to it was reading 2°C lower. Same water, both of them sitting in it.
The glass thermometer on my tank had been reading 2°C below the real water temperature. Shocked.
I had been keeping the tank at 25–26°C going by that thermometer. The sensor says 27–28°C. So I have been growing corals in water 2 degrees warmer than I thought.
It is such a simple thing that it never crossed my mind it could be wrong. A glass tube with a printed scale and red liquid moving up and down — I had never once questioned it.
A while ago I added a temperature controller, set it to 25°C, and wrote a whole post saying the temperature swings were sorted out. But that 25°C was measured with this thermometer. Going by what I know now, the water was actually at 27°C.
Summer is worse. I came home from work, saw 27°C, got worried and added a cooling fan — and that reading was really closer to 29°C. Putting the fan on and telling myself it was fine now, that part bothers me.
If I put my hand in the water I cannot tell 27 from 29. It just feels lukewarm.
It also makes me look again at why the leather coral recovered while the torch stayed half-hearted. The torch is the one that handles heat worse. PO4 dosing, carbon — I kept looking for the culprit somewhere else, and it may have been the temperature all along.
For those months the torch was sitting in water I believed was 25°C.
I am not certain, of course. I changed the dosing, the carbon, and the controller around the same time, so I still do not know what did what. The one thing that is clear now is that the temperature was never the number I thought it was.
From what I read, cheap thermometers being off by this much is common enough, glass or digital. I did not get an unusually bad one — that is just what they are. Not knowing that was on me.
What I do first
The urgent part is dropping the controller setting by 2°C. If I leave it as it is, the tank keeps running at 27–28°C. In summer that matters more.
Putting the thermometer in ice water would tell me whether the 2 degrees is a constant offset or something that only shows up at certain temperatures. Have not done it yet.
And I need to go back over the torch. If it was not recovering because the tank was too warm rather than because of PO4, the last few months read differently.
If you are going by a single glass thermometer, hold something else next to it once. It gets annoying when they do not match, but that beats not knowing.
I did not take the glass thermometer off. Thought about it, then left it. I know it reads 2 low, so I just add them.
