Melosu, I know we've disagreed on this issue before so I want to get my opinion out clear and simple. I agree with you. Keeping horses in a dedicated tank is extremely important. I think most LFS's mis-represent horses and their needs so there is a lot of confusion out there. I hear about people having one horse in with other fish and they tell me "it's ok, I've got a really huge tank..." The main reason I pushed to get this horse thread is to have a discussion just like this, so people can be informed about the needs of horses BEFORE they impulse buy and then, unfortunately, kill their horses so I'm so glad we are putting this information out there.
Now, that that is said, I do think there are a few situations that will allow people to keep a non-syngnathid (seahorse family fish) in their tank. Here's when I think it will work. ONLY if you've got a well established tank, filled with macro algae and non-stinging corals. Good cleansing system. Normal tank lighting (no need for extreme T5's or silver halide) and places for the horses to go to get out of the light if they want. Tank temperature to accommodate your species of horse and you've got horses that are hand fed 2-3 times daily. Hand feeding means you either target feed each horse or you make a "feeding trough" and you watch as each horse eats from the trough. Horses are slow, pathetically slow eaters. Even a vegetarian fish will out-compete a horse for food. If you have a good supply of macro algae then you probably have more than the average supply of pods in your aquarium. If you target feed your horses then you know they are eating. You will look at each horse several times a day and know they are getting the nourishment they need. If you do decide to add a fish it should be mature and you should be certain of it's personality. There will always be the risk that the fish will "change" it's personality and start picking on the horse so you'll have to be prepared to deal with that. Only with care and consideration would I suggest adding a tank-mate but it is, in my opinion, a possibility.
It also depends, of course, on the species of horse. I would never suggest a tankmate with dwarf horses (H. zosterae) and there are many species of horses that have specific temperature requirements that would prohibit adding tank mates. H. reidi are more temperamental types so you wouldn't want to make them uncomfortable. Most H. erectus are extroverts and personable enough that they would do well if the above conditions are met.
Final word: Tank mates with seahorses is NOT a good idea. Keep the horses in a dedicated tank. They love to be in groups so get a tank big enough for a whole herd and their personality will make you fall in love. FISH ARE BORING: GO SYNGNATHIDS!
Catherine
