Receding Hammer

[mickeyfish]

Any idea what could cause a sudden hole in this hammer coral? It had been doing great for the past year and a half.

Params:
Alk: 10.5
Calcium: 460
Phosphate: .1
Salinity: 1.025
Temp: 79

Only recent change was I started using Acropower and Reef Roids weekly to try and color up a faded Moniopora.

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[Uncle99]

Something "munch" on that?
Looks bitten to me.

 

[mneddy]

I've had pepermint shrimps do the same thing. It looks bitten.

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[Sk8r]

Does it have any neighbor that has grown? Sometimes two corals slowly get too close and then something tees it off.