Actually, the more branchlike things in the tank, the better... the horses spend about 90% of their time hitched to something, and they need various spots around the tank to hitch to, in different flow and lighting conditions to be "happy". I agree with the stinging part.... If your corals sting at all, they're not right for a seahorse tank.
The biggest thing to consider is that Seahorses will need to be the reason for everything else you do in that tank.
First off, Taller is better than wide, since they are vertically oriented animals. I started with a regular 29gal, and it was enough to get them happy and mating, but they are better served with more height then length.
The other fish (if any) will need to be very slow moving and cannot be aggressive feeders who will steal the food from the seahorses. Gobies are great companions for seahorses and come in many shapes and sizes.
They don't tolerate intense lighting very well, so it's reccomended that you only use up to power compact lighting, which will limit the types of corals you can put in the tank. Mushrooms and zoos are great, and most Gorgonians are IDEAL for seahorses (branch city, baby...). They need places in the tank where they can get out of the light when they need a break, so you'll have to do a lot of creative positioning to create good hitching spots in various environs within your tank.
Flow is a concern... in general, you need a low flow rate, or more precisely a gentle flow rate in the tank. MOST seahorses prefer a slow gentle current, and this will help food move by them slowly, since they hunt, target, and snick very slowly. I will say that two of my seahorses LOVE the current though, and spend a good part of the day hitched onto something getting blown around.
Feeding is a big issue, since they prefer to have their food moving when they hunt it... but they can be fairly easily trained to accept frozen food, though whatever you feed it needs to be enriched with a vitamin/fat/nutrient supplement. With seahorses, I found both through research and my own experience, that since when they eat a lot of the food particles get pushed out their gills, that this leads to an excess of waste food particles in the water which you really cant do anything about it, so a good skimmer, phos reactor, and a SERIOUS cleanup crew are vital.
Many keepers feed twice daily, but many others feed only one big meal... The thing is, they don't have stomachs, so they need something like copepods or whatnot to snack on between feedings. It is also recommended that you don't go more then 48 hours betwen feeedings... if you are using a nutritional enricher (which you should) you should have one fast day per week, and you can use that to your advantage to go away fro a weekend... feed on friday, skip saturday, feed on sunday. But any longer than that, you're going to have to have someone come over and feed the tank.
Physics and tank arrangement are very important. They need room to perform their courtship and greeting rituals, which means they need at least a few places where they have a complete vertical path from the bottom to the top of the tank (since mating occurs with them traveling vertically from bottom to top and back over and over) and they need some floor space across the bottom of the tank from side to side for other dancing displays. You have to make sure that the vertical spaces are not interrupted by extreme currents which would blow them around while trying to mate and make them drop the eggs.
Color of the horses is determined by both emotion and environment. ALL horses change color. Just because it is one color when you order it or bring it home, does not mean it will be that color tomorrow (or even 5 min. later). The farms raise them in colored environments, so if you order a yellow one, it's because it was raised in a yellow tank, with yellow walls, and a yellow hitching post, but as soon as it gets into your tank, if it's mostly live rock (which is brownish) it will try to blend in. That being said, there are times when you will find all sorts of colors in your herd, and the more colorful corals, hitching posts,e tc that you have will influence them to change their coloring... note I said influence, because you cannot control it. Lots of people spend crazy money on a certain color seahorse... and 10 min after being in the tank it's something else. After a while, when comfortable they will settle on some general coloring they like, but still change when it suits them. I have one that was dark brown for a long time, and now is generally bright yellow.... I have one that tends to be pumpkin and white stripes, but the past week has been mostly black.... Another tends to yellows, but when he's pregnant turns green..... So the moral is you will see lots of coloration if you provide stimulation in the environment, but don't go in thinking "I want 2 pairs of bright yellow seahorses" Or "that red one that costs $400 would look great in the tank"..... go in thinking "each day my horses will show me new color patterns", lol
On choosing your horses, there's a few different common varieties, but I would reccomend starting with "H. Erectus" though some people start with "H. Kuda"... most of the other varieties have tougher keeping paramaters for your tank. I would personally not purchase a seahorse from a LFS. I would order from a seahorse farm that is breeding hearty strains that are tank raised and eating frozen foods. Many seahorses you find in LFS' actually are coming from the farms that raise the seahorses for asian medicinal purposes, and there's not much quality control and the strains are weak from inbreeding. Personally, I use "Ocean Rider" (http://www.seahorse.com">www.seahorse.com</a>) and have nothing but great things to say about their livestock and customer service. There are others out there, but... you're really taking a chance if you buy from your LFS, and.... why take a chance, when you can get guaranteed health, hardiness, and eating behaviors.... (and the ones from ocean riders have been bred to have great pigmentation possiblities)
so.. are they "Hard" to keep? No harder then keeping other fish going, but a bit more specialized i'd think. They some medical problems that can occur, such as "gas bubble syndrom" and various others, but they can be dealt with.
I'm a novice still, but after keeping 2 pairs in the 29gal for two months i moved to the 90g, and now have 4 breeding pairs. They are doing great, starting to breed (2 broods so far, and three pregnant males at the moment)... and I tell you what... I LOVE them, lol. They are just too cool. But the moral is, if a newbie like me can be making this work so well, it can't be that hard.
Just pay attention to them and do what needs to be done. Takes me aobut 15 min of work on the tank each day, 10 min for feeding, then an hour or two on the weekends for water changes and maintenence.
If you would like to see my tank and/or talk about keeping them, I'd be more then happy to tell you what I know. Sorry this is so long, i cant stop talking about them once I start! Let me know If I can help you out or show you the herd!
... I was actually coming on here now to just revise that part, lol..... In my tank I use a large bright yellow rubber staghorn, and a blue plastic octopus coral, and a red something or other, lol, as centerpiece hitching posts, but also have a few live gorgonians which they LOVE, as well as a few patches of artificial sea grass and long frilly plants. They love hanging out on branchy caulerpa as well, and a few of them like to hang on my branchy live rock... They all have different personalities and likes and dislikes, so my point is to provide them with a varied selection of hitching posts and lighting/flow conditions, because each one has different wants.
