Depends on how much dry rock you add compared to how much cycled LR you have. We added two 5 pound dry pieces to our 230g which has 200 pounds of cycled LR and didn't notice anything. Someone else I know added about 15-20 pounds of dry rock to their 40 pounds of LR they had and their tank went through an ammonia spike.
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One more fish should be ok?, right!!!  - Laurie
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