To re-emphasize what the others have said, Please, PLEASE read some good books on Saltwater fishkeeping. Btw, you didn't do Everything wrong; although your local fish store is evidently staffed by idiots, you obviously tried to get advice before you started on your saltwater experience. Some people don't even bother asking anyone and end up wasting alot of money, and worse yet, needlessly killing alot of fish... Also, if you haven't already done so, when you replace your gravel with sand, you have a few options. If possible, avoid going back to the same store to purchase it (at least, don't seek any more advice from them). When it comes to sand, you can get "live" sand or dry ("dead") sand. The live Sand would, in my opinion, help you more, because live sand contains many beneficial types of bacteria and other microorganisms. This will greatly speed up the time it takes for your tank to "cycle". You can also go with dry sand, but don't just pour it into your tank; it contains ALOT of dust which would cloud your water for days. Take small amounts of sand at a time, put it in a bucket with normal tap water and swirl it around. Pour off Just the water, and add more water. Usually, doing this 3 or 4 times is enough. You won't get rid of all of the dust, but doing this should help. Additionally, as the others mentioned, a few pieces of Live Rock will greatly enhance the stability of the tank. Believe it or not, in some cases, the combination of the sand bed and live rock is all the filtration needed, so that no additional mechanical filtration is needed (also known as the Berlin Method, I think). You should NOT try this method; it's just for an example. Just a few more things. If you do decide to use tap water, don't pour it directly into the tank without treating it with some type of dechlorinator. Purchase a hydrometer, to measure the salinity of the water before you add it to your tank. If you need to add water due to evaporation, don't add Saltwater, only freshwater. This is because the salt stays in your tank when the water evaporates, so that if you do add saltwater each time you top off the tank, the tank will get saltier and saltier, eventually killing your livestock. Lastly, don't mix the saltwater in your tank; always mix it in a bucket that is ONLY used for your fish tank. Never use a bucket or any supplies that have been previously used for other things, like housecleaning. You don't use a gravel suction in sand, either. You should try to avoid disturbing your sand too much. It's better to have good circulation in the tank with some critters that can clean up anything that falls to the floor of the tank (like hermit crabs, snails, or some types of starfish) I apologize for so much information, but I enjoy this hobby alot and don't like to see others get discouraged before they even start. I don't know where you live, but you might also see if you can join a local fish club, especially one with an online forum. In San Antonio, Tx, we have MAAST (Marine Aquariust's Association of South Texas); it's a great club and the online forum is an outstanding resource. A local club will put you in contact with folks who might be able to share some of their experiences too. I'd say ask your LFS if they know about a club, but considering the advice they've already given you, I'm not sure what kind of answer you'll get. Regardless, try not to get discouraged and welcome to Saltwater fishkeeping and this Forum! Good luck, Moonrs
