Unless it's the tiniest crab around, only crabs I have are two emerald crabs and 3 hermits, they stay under the rocks, until the Halides are off then they come up and pick at the gha issue.
My parameters are Mg 1360 Kh 9 Ca 430
Salinity is 1.026 Ph 8.2 Temp 78
0 Ammonia, 0 Nitrite, 0 Nitrate...