Originally Posted by Shirtlessbill
Low flow, very clean water.
Low flow is actually a pretty common misconception. Seahorses do best with a moderate to high flow. I personally wouldn't go over a 20x turnover rate, but I know people have done more. My goal is 10-20x the overall tank volume. This also helps with keeping crap from building up in "dead spaces". Just be sure to cover all in takes and overflows, and have some sheltered areas that are out of the direct line of powerheads and returns.
Most species require a minimum of a 29 gallon aquarium for a pair, and add 15 gallons for each additional pair.
Price is $40-$150 for true captive bred seahorses, with $60-70 being average. One concern are "tank raised" seahorses - seahorses that are imported from overseas and grown in natural seawater. They tend to be cheap (some times as low as $26 on Live Aquaria), and far too small. They have a very poor survival rate in captivity.