New clam

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What does a less than fully open mantle suggest ? Is it because it’s using its energy to attach the rock or something else
It’s 2 days now and the first day it fully opened

It can be a few things, acclimating to a change in lighting (intensity, spectrum), flow, parameters, etc. It’s normal for a clam to take a few days sometimes to acclimate to a new environment, but if it persists beyond several days to a week, then there could be something else going on. The best thing to do is allow the clam to adjust and put it in the desired (appropriate) spot and leave it be. You should be able to up the lighting no matter where you put a clam. If a clam requires more light and you put it in a spot where it’s maxed out, there’s nothing you can do to provide the clam with enough light without moving it, which could potentially stress it out. This is why it’s so important to plan ahead and put clams in appropriate areas that will have a surplus of light.