This is what our tank looks like so far....

[canadawest]

You're not alone Em, my tank has been running for almost 2 years now, and I also find my sandbed decreases in size slowly over the year. Perhaps 1" per year is a rough guess?

I just added 1-1.5" of sugar sand on top of my existing fine crushed coral and now it looks great again. We'll see how long that lasts.

But... In my little 12gal Eclipse tank which has been running about the same amount of time with the same fine crushed coral, no sandbed loss.

So perhaps your tiger-tail theory is sound as I have two as well as a pink/black hawaiian cuke in my main tank?