Help please!! ASAP!!

[Spot On]

We added a rock to our 20 gallon...only thing we did. 2 hours later we checked on it and our long nose hawkfish was dead and the 2 clowns struggling. Were the only 3 fish in the tank and only added that 1 rock. Were able to save the clowns. So the rock depleted the 02 or introduced something. We had the rock sitting in a bucket of saltwater for about 3 weeks before introducing it. Sooo

I am not questioning that something happened, but I also do not see how a rock rapidly consumes O2. No natural mineral rock will do that.

What sort of rock was it?

Not sure what kind of rock. Got it from lfs. Its in the 180 without trouble. So not sure exactly what happened.

 

[The Ugly Phase]

Can't adding rock to a tank cause another cycle? or am i making that up maybe ammonia spiked and that killed the fish @Spot On

 

[Spot On]

Can't adding rock to a tank cause another cycle? or am i making that up maybe ammonia spiked and that killed the fish @Spot On

Good question

 

[Randy Holmes-Farley]

Can't adding rock to a tank cause another cycle? or am i making that up maybe ammonia spiked and that killed the fish @Spot On

Not likely, unless it is covered with lots of dead stuff that rots faster than the existing filter can keep up.

Rock that was recently bleached and not neutralized, or which got exposed to toxic chemicals, or which brought in diseases might be possible.