Eventhough you may have metal halides HID lamps, 20000K does not provide enough PAR for the clams! Even if you put clams near the surface of your tank! Crocea and Maxima are grow in shallow waters!
Here's my experience. I had 7 croceas and 4 maximas in my 8ft tank for two and a half years... using 3x10000K Ushio Aqualite metal halides. All supercoloured, super rare collection. Then one day I saw a tank (friend) using 20000K lamps, which has a blueish colour, I thought... wow, that was so beautiful compared to my yellowish-greenish tank even without blue actnics. So I switched all 3 bulbs.
Within the first week, everything seemed normal! Then in the second week, all clams died simultaneously within 2 days, leaving only one alive-located at the corner of tank closest to window(sunlight)! ALL OTHER CORALS ARE DOING GREAT! Only clams died. I had no explanation! Gave up clams for a year plus.
Then, I setup a new tank, thinking that the cause of clam wipeout in my previous tank was a disease outbreak. This time, I tried 2-3 croceas until I gave up. BTW, using 20000K. And I said, I did not have a problem when using 10000K, and I tried reluctantly due to the unattractive color. Now having 7 clams for 8 months already.
So, please spread the word. Clam tanks NEED 10000K lighting! Learn from my stupid mistake.
Here's my experience. I had 7 croceas and 4 maximas in my 8ft tank for two and a half years... using 3x10000K Ushio Aqualite metal halides. All supercoloured, super rare collection. Then one day I saw a tank (friend) using 20000K lamps, which has a blueish colour, I thought... wow, that was so beautiful compared to my yellowish-greenish tank even without blue actnics. So I switched all 3 bulbs.
Within the first week, everything seemed normal! Then in the second week, all clams died simultaneously within 2 days, leaving only one alive-located at the corner of tank closest to window(sunlight)! ALL OTHER CORALS ARE DOING GREAT! Only clams died. I had no explanation! Gave up clams for a year plus.
Then, I setup a new tank, thinking that the cause of clam wipeout in my previous tank was a disease outbreak. This time, I tried 2-3 croceas until I gave up. BTW, using 20000K. And I said, I did not have a problem when using 10000K, and I tried reluctantly due to the unattractive color. Now having 7 clams for 8 months already.
So, please spread the word. Clam tanks NEED 10000K lighting! Learn from my stupid mistake.