Hey I had 3 seahorses in a 10g tank for over a year and they required minimal effort on my part. As far as a cleanup crew this is what worked for me: about a dozen or so small hermits (my rule of thumb is one hermit per gallon), a few small snails (probably not all that necessary), one sandsifting star, and one cleaner shrimp. I had 10 pounds of live sand and a few pounds of high quality live rock- its worth the extra cost because the live sand and live rock keep the water exceptionally clean. I had a small whisper filter (I think a whisper 10 or 20) that I ran on its lowest flow but I put bag of activated charcoal in it instead of the pads it came with...the charcoal can be rinsed as needed in cold water and can be changed every couple of months or so. To be frank with you although a skimmer would always help out its very hard to find a good skimmer that actually works for such a small tank, most of the really good skimmers are for big big tanks and the rest are crap.
From my local store I also got a handful of that green plant you see in the pic (I'm blanking on the name right now) and I planted it into the sand. Because of the low water flow this plant added the necessary oxygen to the water and reduced nitrates from uneaten food. I essentially kept the hermit crabs nearly starving so that way they would never leave any uneaten food hanging around.
I was told from several sources that seahorses need to be fed live food daily because their digestive systems are so delicate and if you miss a day they will die. This was not true for me at all. MAKE SURE that before you buy a seahorse you ask what it is eating and ask to watch it eat. If you do not see the seahorse actually eat do not buy it. Seahorses are VERY finicky eaters I fed my seahorses both live brine shrimp and also frozen mysis shrimp. Eventually all three of my seahorses switched to the frozen mysis shrimp, which has many more nutrients. Also it got to the point where I could feed them one every week or two and they were absolutely fine, they would eat well but certainly did not need to eat daily. I think I may have just had particularly good luck/fish karma, but this is certainly why its important to really watch them in the store before you buy them.
As far as lighting it doesn't really matter. I had just a cheap fluorescent bulb which was enough to keep the live rock and the plantlife alive, but the seahorses themselves don't need any special lighting. The other thing I had in the tank was a lot of things for the seahorses to hang on. They love to wrap their tail around anything (including each other) and then try to eat things that float by. Make sure the water flow in the tank is low enough so that there is no real current because seahorses are very weak swimmers. Definitely no blower because that flow will overpower them for sure.
Also I put in a purple linkia starfish just because I like their color (You can sort of see him in one of the pics on the upper right part of the tank by the filter). They are unnecessary and hard to keep and if I were going to do it again I probably wouldn't put one in there but its up to you
So again, the most important things were the sandsifting star, a cleaner shrimp, plenty of hermits, good live rock and live sand, and anything for the seahorses to hold onto. Feel free to send me an email with anything else I might be able to help with
dgottli1@du.edu
-Dave
