55 gallon reef aquarium in which you have had 3 pipefish they have all disappeared without a trace do pipefish bury themselves in the bottom the bottom substrate is crushed coral.?

No, seahorses do not bury themselves. Chances are they have died and have been consumed by something in your aquarium. If there is rock it is possible they are hiding in the nooks and crannies. However, many pipefish species are difficult to maintain, so in all likelihood they have perished.