You're right about them bitting everything at least once lol. My Blue Hippo Tang favorite dessert is blue cespitularia. This is a picture of it in my Coral Quarantine Aquarium, the long white one. Yeah, it doesn't look like that any more lol.never had any of the larger tangs but you will definitely need a bigger tank at some point, these guys get so big they will have trouble swimming once they reach like 8+ inches. Now i get the nitrates too, these guys eat and crap a lot so its probably getting bound in the sand.
If they want to nip at the tridacna clams, thats weird, but will prob just be the appetite of the individual fish and not much you could do about that. I find my tang will try to bite everything at least once to see if it likes it anyway