Well...you'll need a sump of course. You can buy one ready to go or make one from a smaller aquarium or other inert container. You'll need a return pump to pump water from the sump to the tank. You'll need an overflow of some sort to get water from the tank to the sump. You'll need a good heater...I recommend the Won Pro Heat II in a 150 watt for a 25
gal tank. If you go with one of the less expensive glass heaters (Ebo Jager), I'd get two smaller ones instead of one large one. A skimmer is recommended for any
SW tank because they remove lots of dissolved organics before they can break down in the tank. For a 25
gal tank, my choice would be an Aqua C Remora (Urchin for in sump use) with a MJ1200 pump. You'll want 25-40lbs of
LR. You'll need test kits (I recommend Salifert or Seachem) for ammonia, nitrite, nitrate, alkalinity, calcium, and pH. You also need an hydrometer to check the salinity of the water. Since this is going to be a
FOWLR tank, lighting isn't that critical. I would have at least two
NO bulbs...one daylight and one actinic. You may also want to have a powerhead or two in the tank for flow although,
IMO, 400-500gph will be more than enough for a tank this size. One last thing...smaller
SW tanks are more difficult to keep up than larger ones. The reason for this is that you are dealing with a small volume of water and parameters can change very quickly...much more quickly than in a larger tank. Stocking levels in
SW are much more conservative than
FW. My 20gal tank houses a small clown and a Banggai cardinal...I wouldn't add any more fish to it. I would keep my fish load to 4-5 inches (adult size) in your tank. I've probably left something out, but I'm sure someone will chime in with some more suggestions. Read the articles on this site as well...there is a lot of good info there.