That's the trouble with seahorses, they are extremely fragile and all conditions need to remain just about perfect from them to survive in a closed system. Something simple as leaving uneaten mysis in the tank overnight can cause an ammonia spike that will kill them instantly (same for many fish). My tanks are bare bottom so I can see if anything is lying around the bottom after feeding. I also have a good cleaning crew but they aren't especially happy about living in a bare bottom tank (think ice skating hermits). Very sorry about your loss and I hope you have better success with your next ones.