Angie,

Are you going to have a sump? Refugium? If so, just put a DSB there and keep your display as a shallow sand bed.

Now, with that said, the reason for the 3X height "rule of thumb" is to ensure that there is enough head room for their "happy dance" and egg transfer. If you don't much care about breeding, no harm..no foul. EVEN if you do have dreams of baby Erectus in mind, you MAY not be in too much trouble... let me 'splain....

First of all, MOST seahorses in captivity do NOT get as large as those in the wild. There are many theories, many reasons, no matter...it's just as it is.. they tend to be at least an inch or two shorter than their wild cousins. So, keeping that in mind, you may have some "fudge factor" room here.

Second of all...that 3X rule of thumb is just that...rule of thumb. Here's a funny story.... I was TRYING to give one of my males a "break". His mate would deposit eggs just hours after his giving birth. I figured, I'd put him in an offline refugium that I had running (10 gallon). I did this, and he loved the digs, but was a little "crazy" swimming back and forth as if irritated. I also found that his "gurl" was doing the same in the display tank. Okay.. who could blame them... who am I to split up "true love". So, into the fuge momma goes. I said to myself "Self...there's not enough space for them to mate in there.. no worries". Guess what...

Not only were they successful in doing an egg transfer in a fuge with DSB and VERY little water/head room, but they did it in a record 10 minutes.

So... the bottom line is this... we are RESPONSIBLE for creating the BEST environment for our charges. What is "best" is really a matter of interpretation. If you set up the new tank with a couple/few inches short water column and the fish seem happy... then leave well enough alone. I'm sure you'll be putting in young-ish fish. They will have plenty of room for now. If you notice that there is a problem, slowly start removing some of the sand, over a period of time, and reduce it to a better height.

On to macro.... macro will be plenty fed with the nutrients in your water. They DO NOT eat detritus and such from the sand. They absorb waterborne nutrients. They DO NOT need any more supplements than are in a good commercial salt water mix. They thrive on the compounds that you want them to take.. ammonia etc. Should you care for them like you do a fresh water planted tank, and feed them, they will eagerly accept these additives and therefore not be the "filtering machine" they can be as it pertains to the nutrients in your water. Furthermore, by adding ANYTHING in there that may be "good" for the macro, will also benefit any UNDESIREABLE algae in your system.

Your best bet is to let the lighting and the natural nutrients feed your macro. If you don't get expected growth (vigerous for caulerpas, slower for macro like chaeto and gracilaria) then we can talk again. Otherwise, I'm sure you will be pruning the stuff MORE OFTEN than you want to be

Dave