Watch your fish very closely to see if they are the culprits.
I also would not be using conditioned tap water unless you know exactly what is in the water first... there are things like copper, zinc, flouride, leftover medications, chlorine, chloramine (which is not removed by some tap water conditioners) and a whole host of other stuff. Use RO/DI water when possible - from your local Walmart would be a fine temporary fix, but designing a RO/DI unit to fit the needs of your tap water conditions is probably going to be key to long term success.
Copper, even in very low and trace amounts, can kill inverts. send a sample off and have it tested.