My clam went downhill in 48 hours

[Cobalt]

After bringing it home from C&C’s I noted a pretty purple sponge growing on it that has slowly grown over the past 5 months. But not into its shell and it never seemed to bother the clam.

 

any ideas or things I should check? Params are all ok other than my alkalinity wandered a little high at 13 now but all the other tank inhabitants are happy.

[Cobalt]

Before

[Cobalt]

I have been reluctant to scrub the sponge off because they I do read they are toxic and thought it was better to leave alone and the clams mantle was touching the sponge and seemed fine so it let it go. 
 

other clam in the tank is happy as a... clam

[Higher Thinking]

What's your par levels down on the substrate? The other clam looks to be situated much higher. The clam on the bottom doesn't appear to have a significant growth ring of new, white shell. You should always see a fresh rim of new growth. That's the first thought that pops into my head. I doubt a sponge is doing anything, but I honestly can't speak to that.

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[Cobalt]

I need to check out the par meter but the clam is directly under an arm mounted gen 3 radion XR30 at the bottom of a 20in tank.  I also have a blue LED light bar for some additional PAR for the edges of my 36in tank.

 

Removed him. No evidence of predators/worms. Maybe I shouldn’t have had him in the sand bed. 

[ocboat]

2 hours ago, Higher Thinking said:

What's your par levels down on the substrate? The other clam looks to be situated much higher. The clam on the bottom doesn't appear to have a significant growth ring of new, white shell. You should always see a fresh rim of new growth. That's the first thought that pops into my head. I doubt a sponge is doing anything, but I honestly can't speak to that.

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I agree 100%.  Clams can look great one day and then dead the next.  It looks like the clam has not been growing for quite some time.  I've lost a few clams just like this over the years while others have thrived.

[danlu_gt]

Healthy clams and non-healthy clams mental look almost identical.  By the time it show signs of issue, it's almost always too late.  Like what Andrew said, healthy clams should always have new white shell deposit.  If not, it cause be not enough PAR, not enough nitrate/phosphate, low Ca/ALK, or parasitic snails.

Also, I don't think maxima and crocea clams like to be in the sand.  

[Cobalt]

Thanks for the comments. The clam did show growth at one point. I forgot to look for that more recently. I’ve been keeping my phosphates at .05 or below, nitrates at 2mg/L. Ca 420 and Alk at 11. Have heard about the sand thing.