Regarding the sand-sifting starfish, I would like to know as well, so I'm curious to hear if anyone else responds. I've been researching them, and the most common comments I see is that they typically will starve after a few months to a year. With only an inch of sand, I would bet there's a good chance it would starve unless you routinely fed him directly (and made sure he ate what you gave him, and didn't get robbed by your other tankmates
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pH, Alkalinity, and Calcium (as well as Magnesium) are all related to one another, ether directly or indirectly. Pasfur is very educated in this 4-way relationship, but I think I can give you a crash-course: pH and Alkalinity are directly related, where higher Alkalinity implies more ability to buffer pH and keep it up where it should be (so pH won't fall as much with higher Alk). Alkalinity and Calcium are also related, but I don't know specifics on their relationship... finally, Calcium and Magnesium are related. So for example, pH and Magnesium aren't directly related, but they are indirectly because pH relates to Alkalinity, which relates to Calcium, which relates to Magnesium. Basically, it should be your goal to monitor and keep all of these guys in stable ranges. If one or more are out of whack, or varying/unstable, then it can cause problems or instability with the others
So the change in pH that you see MAY be related to the temperature difference, but it is also being affected by the swings in your Alkalinity. My guess as to why temp might affect it, is because temperature affects the metabolism of the things living in your tank, which affects the pH of your tank. For example at night, when most things are sleeping/inactive, the pH of the tank naturally drops. However, since your tank is so lightly stocked at this point, I would guess it is being more affected by the Alkalinity than the temp (just a guess though!)
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