Starting a saltwater fish tank.?

First post is correct. Even your smaller clowns (ocellaris and percula) require bare minimum around 30 gallons. Also damsels are very aggressive and you would have to get to rid of them after your tank has completed its cycle, however your live rock has not been given ample time to cure in the tank. It doesn't matter what quality the live rock is, it must be given time to cure properly. Your tank would have cycled properly with the live rock alone, as the micro-organisms died off and produced a food source for the nitrifying bacteria. Albeit slowly, none-the-less it would have cycled. Both tangs are out of the question for this tank. A Yellow can be kept in a 55, although as the first post said I would not keep in a Blue Tang in anything less then 125 gallons. I have 2 blues in a 125 gallon reef, and they still seem short for room at times. Ditch the bubbler as salt creep is not something you want to clean up constantly and buy some powerheads, hydor-koralia nano pumps are good, as are maxi-jets. Maxi-jets are more direct where-as the hydors have a larger range of circulation. As far the hardness is concerned more info will have to be supplied. Seahorses can be kept with smaller, non-aggressive gobies, however if you want seahorses that is all you will able to keep with them. Also they are advanced aquarists species and have special requirements which are relative only to themselves. Slow down and be patient. Nothing good happens quickly in this hobby and the best thing you can do is to let things happen on their own. If you keep moving too quickly your problems and issues will merge in on one another and it will become more difficult to backtrack to the sources of any of the issues. This will result in a complete tank collapse and you will have to restock. Get a marine beginners book off of amazon or at a local bookstore and read it thoroughly. Research is required in this hobby and it will pay off. Truly best of Luck