Re:Converting a 26 bowfront reef to a seahorse tank

Pete Giwojna

Dear Louis:

The usual rule of thumb is to provide up to 1-2 pounds of live rock per gallon if it will be serving as the primary means of biofiltration for the aquarium. That amount of live rock will provide adequate levels of both nitrification and denitrification for the tank. You can simply select the precured live rock you find most attractive at your LFS and add enough of it to create interesting rock formations that are aesthetically pleasing to your eye. Use enough rock to create some interesting caves, arches, ledges and overhangs.

When it comes to the attractive, natural landscaping for seahorses, I like to create interesting rockwork formations along the back of the tank and along either side over a thin layer of live sand, with a lush bed of assorted macroalgae (marine plants) in the open central portion of the aquarium.

I like to use a Greek amphitheater or horseshoe shaped arrangement of live rock as my main template for my rock formations, since this leaves the foreground of the aquarium relatively open and uncluttered. I prefer fairly brisk water movement in my seahorse setup, so I like to create a taller rock formation atop this foundation about a foot or so down current from the strongest water flow to intercept and further divert that strong full of water, creating eddies and slack-water zones where there is relatively little water movement down current. Small powerhead(s) can be positioned as needed to prevent dead spots and assure there is good circulation throughout the tank.

But for starters, just lay the foundation of your live rock roughly in a U- shaped arrangement and perhaps arrange a second player of live rock to create a few additional solidly- anchored caves, ledges, overhangs and/or arches.

The U-shaped or amphitheater-like arrangement of rock leaves the center and foreground of the tank relatively open to provide swimming and courting space for the seahorses. This is where I like to concentrate my beds of marine plants, which serve as natural hitching posts for the seahorses. The bulk of these plants consist of Caulerpa in a variety of shapes and sizes. The Caulerpa consists mainly of long-bladed and plumed or feathery varieties such as Caulerpa sertularioides, Caulerpa mexicana, Caulerpa ashmedii, Caulerpa serrulata and Caulerpa prolifera. The center of the tank is aquascaped with more macros — mostly red and gold species of Gracilaria (Hawaiian Ogo). The result is a colorful macroalgae garden with a very nice contrast of colors (reds, yellows, greens, and brown) and interesting shapes. A tank heavily planted with macros such as these is a lovely sight and mimics the seahorse’s natural seagrass habitat well.

Yes, sir, a combination of trapping and biological control often works well for keeping the population of bristleworms in a seahorse tank at manageable proportions. Arrow crabs (Stenorhynchus seticornis) are natural bristleworm predators. Large arrow crabs can sometimes be problematic for seahorses, but in my experience large seahorses get along well with smaller arrow crabs. So you might want to try a medium-sized Arrow Crab, which will predate small bristleworms and keep their numbers in check.

I would characterize arrow crabs (Stenorhynchus seticornis) as opportunistic omnivores. I have kept them in a number of my aquaria over the years, including a few seahorse tanks, without any problems. They never bothered my Hippocampus erectus at all, but they can be hard on sessile invertebrates in general and I certainly wouldn’t trust them with dwarf seahorses. Nor would I trust them with small fish like gobies.

I kept a couple of large arrow crabs in my Monster Bin with a 14-inch African lionfish (Pterois volitans) and a couple of blue ribbon eels, and the arrow crabs proved to be fairly proficient at capturing the live minnows I fed to the lionfish and eels, particularly after the minnows had been weakened by the saltwater. If the opportunity presents itself, they are quite capable of capturing small bottom-dwelling fishes.

Arrow crabs will happily devour any bristleworms they can catch but they won’t eradicate them from your aquarium. Too many of the bristleworms always remain inaccessible to them within the rockwork and sand for that, but a small to medium-sized arrow crab or two can help control the bristleworm population. A fairly effective way to reduce their numbers is to regularly trap large bristleworms after lights out along with keeping a young arrow crab to thin out smaller worms (providing there are no sessile invertebrates in the tank the crabs could harm).

In my experience, small to medium-sized arrow crabs are safe with large seahorses and can be used to help limit the number of bristleworms in your tank. But if you want to try this, you don’t want to pick out the biggest, baddest, bruiser of an arrow crab to do the job! Go with a smaller specimen, keep a close eye on it, and be prepared to replace it with a smaller individual after it molts once or twice. They grow fast and can nearly double in size after each molt.

Remember there are always exceptions to every rule, and large crustaceans are never completely trustworthy. Even the most harmless and seemingly inoffensive crabs can cause trouble under certain circumstances. For example, not long ago I heard from a hobbyist that had been keeping a decorator crab in his seahorse tank. All went well at first and there were no problems of any kind for months until, for no apparent reason, the crab suddenly began to quite deliberately amputate portions of the seahorses’ tails. It was not attacking the seahorses as prey or attempting to eat its mutilated victims, it was merely methodically harvesting portions of their anatomy with which to adorn itself! It was simply doing what all decorator crabs do — snipping off and gathering bits and pieces of its immediate environment to attach to itself as a form of natural camouflage. It just goes to show, with crabs you can never be sure how things are going to work out…

For best results, be sure to implement a regular program of trapping in addition to using a medium size arrow crab to reduce their population. A number of bristleworms traps are available at aquarium outlets and can serve as the seahorse keeper’s first line of defense against these prickly pests. Here are some tips from Dee that explain the best ways she’s found for trapping bristleworms:

deemarie1234 <[email protected]> wrote:

I actually caught a bristle worm last night that’s massive!!! Only 4"
long but nearly 1/2" wide! I used a trap for him. I wasn’t going for
him. I knew there was a big one in my reef tank and had seen one
about 7" long. He was the one I was after. My hubby woke me up this
morning to come see the one I did manage to catch.

Besides him though I’ve caught quite a few smaller ones as well. I
like to check my reef tank about 1 1/2 hrs after the lights have been
shut off. Then when I find them I watch them as they flee from the
flahslight and know where they are hiding. I set the traps right next
to their hidy holes and catch them. If they are really narrow the
traps aren’t that good. They can get back out very easily so you
can’t wait until morning to check the traps. You need to check them
periodically through the night, if a small worm is in the trap, empty
it then don’t wait till morning or he’ll be gone. Even the huge one I
just caught managed to get into a hole about 1/8" in diameter to the
food chamber.

I’ve had better success with nylons. I buy knee high nylons and at
night drop a rock or two in the toe along with some food. Then I set
the toe area right near the holes. When they try to get the food they
get stuck in the nylons. In the morning I just pull it out of the
tank and get rid of them.

I’ve also used a turkey baster a few times. If you can get into the
tank quiet enough they won’t really move, squeeze the ball and when
you’re really close to the worm let go. The suction can be pretty
good sometimes depending on where they are.


> Anyone out there with any great successes at catching and
> eradicating your tank of bristles without the use of drugs. This is
> important to me since I have an amazing Coco worm and a few other
> things planned for my reef that will be harmed by the so
> called "wormicides". Ive been using a trap with on and off success,
> actually caught about 1-3/night for the first few nights, and not
> having much success catching the more cautious ones. I think there
> might only be one or two left but not able to coax them out.

The best bait for trapping bristleworms depends on what I have handy at the time 🙂

I found the brine shrimp and sometimes krill (both frozen) work well.
I usually add some to either the trap or the nylons and then take a
small container of water from my reef tank to thaw it out before
putting either the traps or nylons in my tank.

I’ve got two more tricks for ya, First the turkey baster. It doesn’t
really have a lot of suction power. Or at least I’ve never found one
that the ball was on tightly. So I bought a new one and took the ball
off. I wrapped the end with thick clear tape, smae diameter as duct
tape just clear tape. Then when I put the ball back on it was much
tighter giving me much better suction.

Also there is a different type of bulb that is very useful especially
for small worms. They make them for babies noses and you can also get
them anywhere they sell camera equipment. They are used to blow dust
off camera lenses. They are a good size to fit in your hand snugly
and have good suction. Again to make it better there is a small hole
in the bottom on the bulb. I used a piece of a silicone ear plug and
covered the hole. Now I’ve got twice the suction and it works great!!
Eeven though the spout end looks small it really can suck up some
good size worms.

Personally I’m fascinated with the night time world of my tanks. I
completely dewormed my rocks in my sea horse tank. But as for my reef
tank, I kinda like having some brstile worms in there. They really do
clean up a lot of stuff and in tiny little spaces where noone would
ever think would need cleaning. But when I start finding portions of
dead hermit crabs I get a bit cranky. I still can’t kill them though.
I’ve still got the huge one alive and well and am trying to figure
somewhere where I can put him so he can live. I’d love to watch him
closer. I also get a bit cranky when I reach into a tank at night and
don’t watch where I’m reaching. Oops! Got hit pretty hard by a
fireworm one night from a brand new rock. Ended up pulling out three
bristles and got a good sting that lasted 3 days.

Good luck catching your worms, once you start you’ll get hooked on
catching the little monsters 😉


Best of luck modifying your bowfront aquarium to serve as a seahorse tank, Louis! It sounds like a sensible plan and with your background as a reefkeeper, I’m sure you’ll be a very successful seahorse keeper.

Happy Trails!
Pete Giwojna

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