I have a derasa that dang near caused an avalanche....it crawled between two supporting "pillars" of rock and was slowly "wedging" them apart....I just went in yesterday and moved the clam.... and fixed the "rock slide waiting to happen"...
Finding a flat or slightly cupped piece of rock is probably best...something small enough that you can move around and potentially add to your rock work. How deep is your sand bed? If its not to deep you could move some sand away and let it attach to the glass....really easy to move the clam if you ever have to.
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260g mixed reef, 105g sump, water blaster 7000 return, Bubble King SM 300 skimmer, Aqua Controller Jr, 4 radions, 3 Tunze 6055s,1 tunze 6065, 2 Vortech MP40s, Vortech MP20, Tunze ATO, GHL SA2 doser, 2 TLF reactors (1 carbon, 1 rowa).
http://www.canreef.com/vbulletin/showthread.php?t=50034 . Tank Video here
http://www.vimeo.com/2304609 and here
http://www.vimeo.com/16591694