first, watch it clsely, if there is any movment at all, it could be a worm of some type or burrowing animal
and
mounds tha size, could be gas, but it sounds a bit big fro gas bubbles, to me, but could be, but here are a couple of other possibilities too, 1st carib sea is very fine, and if you alyered it very smooth, it will not remain the same your water movemnt will more than likely create a wavey affect over time, adn some places where the powerheads are affecting the current will ctreate small mound and or dig a hole, also things moving around like your anemone will also shift your sand a bit as well, the goby you have, is also a sifter, and will move the sand around, also any snails or worms you have, may do some, for rearranging the sand, as they do burrow(some snails do, yes), any stars, and you could have gotten anything on the lr;
but as far as exactly what, or what combo is doing it, it is hard to say, the only wone, that i would be concerned with, is the powerheads, i would watch the sand real close for a couple of minutes to see if i see any sand grains moving, at all, if this is the case, i would rearrange tehm, a bit, because if they are doing it, they will eventually digup a hole, which you want your sand to remain pretty stable, and undisturbed, any others, unless they get real bad, are not a problem, but actually natural and maybe even good for your system
i had not answered sooner, because i was trying to kick around a bit what would be the best answer, and there are so many possibilities for your answer
HTH
