Check FloridaPets.com for some Sea Lettuce (Ulva). They usually have it in stock. Not sure where to get the seahorses online but that is something that I would try to buy locally if at all possible. That way you can see exactly what you are getting and will likely have a lower mortality rate.
What you want is either reverse osmosis or deionized water (both would be best RO/DI). Getting the best water you can will save yourself some headaches later on down the road. Most grocery stores, and probably Wal-Mart, will have the machines that dispense RO water into your own 1 or 5 gallon containers. It is usually like 25 cents a gallon or something like that. The problem with this water is that you can never be sure when the filters were last changed, maybe 1 month, one year, who knows maybe never. Buying pre-bottled water from the store will get expensive very quickly. If you must buy pre-bottled then look for deionized water. Stay away from anything that has minerals or other additives added to the water. Try to stay away from distilled water as distillation is normally done with copper hardware and copper will kill inverts. Though distilled water would most likely have very low to zero levels of copper, it is just not worth the chance in my opinion. I would avoid spring water and opt for something that says on the label how it is filtered. In the end you will find that it is much easier and cheaper to get your own RO/DI unit, until then I would use the machine dispensed water.
Hope this helps!
