I think I'm having a bit of the same problem, I bought someones 30g tank for my 20L tank with 5 gal sump, after moving the livestock it caused a mini cycle as expected. Over about 4 days the leathers in the tank started to lean over, I thought they would come out of it, but they started to die, I left them in there for 2 more days( there where 6 of the) hoping they would recover, in those 2 days most of the SPS died, starting with a green birdnest, I got the leathers out but the damage is done. Hopefully the rest recovers.