what can cause this

To be honest I dont think You should think too much about the corals. Its much more important to get the tank going. When that is done corals will grow and have a good life.
The first thing to check in a tank is temperature and salinity.

To be sure about the temperature in the tank I would by an ordinary glass thermometer in a LFS for 1 or 2$. Ask to look at a few and take one that shows the same as most of the others. Use that to check Your electronic meters and put in a drawer afterward. Take it up a few times a year and make a safety check.
You need a good salinity meter. I recommend a refractometer but some floating hydrometers and electronic meters are good too. The problem is just to know which are good. A swing arm hydrometer is often very unreliable so dont try that. If You have one - get rid of it fast.
Next are the water values and they are good. But phosphate and nitrate values are missing. They are often the problem with water from LFS.

The tank and light is ok. If the pumps are powerheads and not stream pumps 10x turnover will give about the same circulation as 40x turnover with with stream pumps. If you look at a dust particle floating around in the tank low circulation is about 1 /2 to 2 inches per second. Medium is 2 to 6 and high is 6 to 14 inches a second. The important thing is not what kind of pumps You have but the flow at the corals.

The next thing is the biochemistry. I recommend that You buy a live live rock from another LFS or a fellow reefer to get a living bacteria culture and all the small critters You need to keep the rock rinsed from inside. With another source You double the chance to get a good culture. In a month or two they will have colonized the other rock and after that it is time for corals.
 

IF YOU HAD TO TAKE A REEFING EXAM, WOULD YOU PASS?

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