8.0 is not bad, IMO, and I would do some serious searching on alk and PH before you get too far into buffering it up...there is a complex relationship on it. Search for pH, alkalinity and things posted by Bang Guy. I am sure he will have some good advice. Also time of day is important in your pH reading (early will be lowest, end of day will be highest...8.0 is pretty normal, IMO).
Was the red star a red brittlestar?
Your tank is much too young, IMO for any type of star, which are very delicate when it comes to things like acclimation, salinity (which is a bit on the low side, IMO - 1.026 is more reasonable) and other water quality parameters that tend to fluctuate a lot in a young tank. In addition, long acclimation for stars is important...many will die in the first month from acclimation shock. If this was the red brittlestar, it needs a pristine reef tank with ideal salinity, etc, and a long acclimation (4+ hourse). In many cases, it is doomed.
The sand sifter stars commonly starve to death in tanks in 9-12 months after depleting the sand bed of most beneficial critters. If it ever appears to be disintegrating - losing the tips of the arms, or you notice hermits or something eating it...it has starved to death....common in most tanks under 100g. Some may take to spot feeding, which I encourage trying early on (bits of shrimp, squid, etc).