Derasa's are very hardy, but will get big. My ten your old clam easily occupies a good 1/4 of my open sandbed.
ain't that the truth.
i got my T. derasa about 18 months ago, it was about the size of a golf ball when closed. now it verging on softball size. i love it, but i am also very happy it is tank upgrade time, because this guy is running out of room.
so far i have a T. derasa and a T. crocea, both reasonably hardy in my opinion, given the proper conditions. crocea don't get nearly as large as derasa will, but they are very demanding about the amount of light they receive. so they will require a spot high up in the rock work under very intense lighting.
derasa do well on the sand, with less requirement for intense lighting.
whichever type you pick though, the most important thing is getting a good specimen. you want to make sure the mantle has a good, vibrant, hue to it. the clam should react quickly and strongly to sudden shading. it should have a well defined growth edge under the mantle on the leading edge of the shell. the intake siphon should not be gaping, and the clam should not look "stretched" between its shell.
you will also want to check closely for any signs of pinched mantle (derasa are supposedly not very susceptible to this, but other types of Tridacna are), and make sure you inspect it closely, on multiple occasions, for pyramid snails. they tend to come out at night, so after lights out is a good time to check for them. they can be removed manually, and tend to hide during daylight hours around the byssal opening.
they are a great reason to figure out a QT strategy in advance.
