Originally Posted by unleashed
if your using tap water at least use prime to remove amonias chlorine and chlorimine..they most likely died from chlorine burn .which will effect your fish in a slower manner.oh yeh if you only have fish and LR in your tank at this time you can direct dose the tank itself prime is formulated to be used up to 5X's reg dose for extreme cases.
I disagree with chlorine being the cause of this. It does not explain previous failure of the tank, nor does it explain why the fish are fine. Additionally, chlorine does not remain in a system for long nor is it at fatal concentrations in most cases unless you do a 100% water change with no dechlorinator and immediately add the animals within 24hours. The fish would show rapid signs, IMO, of chlorine poisoning.
It is not likely at all in any way that your Eheim provided too much circulation and prevented bacterial growth on the LR. That is just not how it works. The Eheim has substrate for biological filtration, but ALL SURFACES IN THE TANK from the glass, to live rock (or dead rock), sand, gravel, whatever...grow this bacteria. The only way that you can decrease the population of bacteria is to have a relatively low bioload with little food going in. The bacteria population is controlled by the amount of ammonia in the system. Not that one filter is more effective. If your LR was in saltwater, it remained "alive." Whether it was right to remove it or not is open to question. If it is a heavy metal situation, it might have been the right decision.
I agree that the red slime was caused by water quality issues and lack of circulation in the tank. Possibly lighting as well.
My guess is that something was in your tap water that eventually accumulated to the point it became toxic. Up to that point inverts may have died off slowly, but you would see little in the way of worms, copepods, etc (things that reproduce in tanks). Larval forms are far more delicate - lower concentrations of metal would kill them rapidly. So when the tank was initially set up, you may have seen, like most, lots of copepods and other critters from the rock and their numbers would have declined as time progressed. Eventually, the concentrations would reach a point where they became toxic to most of the inverts. You would see things like snails on their backs still alive but almost paralyzed and unable to try and right themselves.
If heavy metal is suspected, you should do several water changes and may also wish to add a heavy metal resin such as that produced by Kent and the polyfilter as well. But it can be an uphill battle. You may also wish to get a test done of your tap water by a company that will test it for things like copper and iron (some places that sell water purification systems or something will do this).
For perspective: I came to this conclusion on a 3500g reef tank and I WAS using RO/DI (the source was within the system itself). It was corrected in time.