From my experience, it really depends on the type of tank you have. Oceanic and reef crystals and some others for example, have higher concentrations of calcium which is good for reef tanks which have alot of calcium guzzling corals, but can be lethal to say your sea stars in a fowlr and invert tank, because when mixed, the calcium is usually beginning in a "too" high range. I ran out of instant ocean once and used oceanic and my calcium went way above 600...I have fowlr and inverts and two of my three starfish ended up dying because afterwords it took nearly two months to bring the calcium back down...because with inverts you can not drop your specific gravity much below 1.024 and the only way to bring the calcium down is to dilute your water with r/o water, which in turn brings down your salinity...Now that things are back to normal, the chocolate chip star, the only survivor of the three seastars is returning to his normal color and health. Just remember to always test your water mix before adding to the tank and you should be ok.