Once it starts to die you will know it... sometimes they detach from the rock and other times they stay where there at but when they start to go the water will start to foul up and they will have a melted slime lookto them and its not a good idea to expose to air to check but every bra i've had that died had an aweful dead smell to it afterwards.. only thing you can do is give it time keep a close eye on it and if it goes from bad to worse time to chunk it unless you wanna have some pretty fouled up water