[Delphinus]

Well, the answer to all your questions are "it depends" but here's my stab at something maybe a little more helpful.

Assuming you are talking of tridacnid clams..

1. Generally they are not regarded as easy to keep, although some people have luck with them. I am not one of those, I've never been able to keep one alive more than 2 years or so. For me, inevitably, something happens, and they die. So personally, I don't think of them as particularly easy to keep. Tank got too warm on a hot summer's day. The clams died. Tank got overfed when I was on vacation. The clams died. My calcium reactor sprung a leak, and was offline for a little while, whilst I got it fixed, and the clams died. It was a cold winter's day when I got the mail-order shipment in, and the clams died. One of them got a disease, and died, and shortly after, all the other clams died. I dosed too little DT's phyto, and the clams died. I dosed too much DT's phyto, and the clams died. I got to the point where I believed if I were to so much look at the tank in the wrong way, I could kill a clam.

That being said, the species can make a difference. In order of "most picky/finicky/sensitive and least sensitive" to "least sensitive and most forgiving" the ratings seem to go: T. crocea, T. maxima, T. squamosa, T. derasa. I'm not sure where T. gigas fits in there, I'd guess to be closer to T. derasa, but they tend to be too large for most aquariums (most people don't really want a clam the size of an ottoman).

2. "Better" is a subjective term. If you mean "pretty and colourful" then T. maxima and T. crocea tend to be the favoured ones. For patterning, T. derasa can be exquisite, as well as T. squamosa. In terms of hardiness, T. derasa is probably the favoured one. If you want something that stays reasonably sized, then T. maxima, T. crocea and T. squamosa are good choices. If you want something that can conceivably get as large as a football, then T. derasa would make a good choice. I have seen, on rare occasion, T. gigas available, but they tend to be less interesting to look at, and these are the ones that you see pictured and they're 1m across. However if anyone in captivity were to actually grow one to that size from a juvenile, I would bow to their superior reef husbandry skills because that would be an extraordinary feat/accomplishment.

3. As adults, they tend to be mostly photosynthetic and rely more upon bright lighting. As juveniles, they tend to need phytoplankton feedings (in addition to lighting).

4. Bright lighting is best. I don't know of too many long term success stories under NO/VHO/PC lighting. Although it is possible that the less-demanding ones like T. derasa may do fine under such lighting setups, the usual ideal setup for them involves halide lighting.

5. It depends on the fish (species). Some fish, yes for sure. Others, no problem. I think butterflies and angels may be the riskiest ones (I don't really know). I've seen several tanks with pygmy angels (coral beaty, flame, potter's), and there were no problems with them picking at the mantles of clams. However I think with angels that it's hit and miss, you may get a "bad apple" and that's all it takes.