I have three Tridacna croca clams in my tank. Their sizes vary between 4.5" to 6". The oldest clam, which is the one in the middle, has been in my tanks since September 2011.
Ensuring that a clam attaches to a surface is sometimes easier said than done. Some people claim that if a clam does not attach to a surface it must be unhappy. My experience suggests that a clam can attach to any surface providing that it is forced to do so in a humane way and will stay attached unless it is removed by brute force, or it dies. I used plastic cards (e.g. an old credit card) to box in clams up to ten days to ensure they they firmly attached to a flat surface. Two of the clams shown below were attached to the flat rock in the picture in October 2011 and still remain attached to the same rock to this day despite a tank move and being taken out of the tank many times with their rock.
Sadly, my clams have been suffering from pyramidellid snails since May 2012. The common advice is to mechanically remove tiny snails on a regular basis, which I did for many months.
However, the snails kept coming back. I then purchased a H. melanurus wrasse, which had a huge impact on the snail population until it sadly died suddenly. The sudden increase in the population in Week 4 coincides with death of my H melanurus.
My experience suggests that even though certain wrasses are very good at removing snails they cannot eliminate the entire snail population because snails evade wrasses by hiding in the sand, in rock structure and even underneath clams. Having realised this I removed the sand bed completely and snail population dropped very significantly. With the help of my latest wrasse, H. hortulanus, close inspection of clams at night time only reveals a very small number of clams once every few months. This breakthrough was only possible when I removed the sand bed completely.
Having said that I am actually very pessimistic about the possibility of removing pyramidellid snails from my tank completely. I argue that once you have these snails, whatever measure you take is not going to go beyond population control. Therefore, what is important is keep the pyramidellid snails number very low, so that these creature will cease to be a thread to clams. This is what I have been doing since May 2012. The fact that my clams have been alive and thriving since the discovery of these tiny snails back in 2012, support my statement.