The most effective preventative for ich is the tank transfer method. It's best done using smaller tanks, 10-20 gallons or so. It's a very simple process, but a bit labor intensive.
- Setup two bare bottom tanks, each with a heater. Same temp, pH and SG.
Note: A small internal filter can be used, or a simple airline to bubble. Don't use an air stone as it can not be reliably cleaned.
- Place the new fish in tank A for 24 hours
- Transfer the fish to tank B, empty and clean tank A, fill tank A with fresh saltwater. Be sure to clean any other accessories (filters, PVC etc)
Note: Some folks choose to use water from their display, which allows the display to receive a nice set of water changes
- After 24 hours in tank B, transfer the fish to tank A, empty and clean tank B, fill tank B with fresh saltwater
- Repeat for 10 days
- Congrats... no ich!
Another good approach, which also works for larger QT tanks (40/55 etc). Setup a bare bottom tank, add an external bio-wheel type filter. Dose the QT with Seachem Stability or similar bioactive additive.
One can then treat with PraziPro, General Cure or other drugs as necessary. If antibiotics are to be used, remove the biowheel and put it in a ziplock for a couple of days. If the QT is dormant, but you wish to maintain the bacterial colony, ammonium chloride is readily available to treat the tank. Adding a few drops weekly keeps the bacteria fed when the tank is empty.
If you are looking to dip your fish, ParaGuard (a formalin base) is an excellent choice. Dips are useful when you don't want to use Prazi on a fish that isn't eating well. Prazi often suppresses appetite.
One of my favorite tools to monitor QTs is the Seachem Ammonia Alert. It will visually react to rising ammonia levels, which are your indicator to perform a water change or add more nitrifying bacteria.
Many folks play russian roulette with their display tanks. Never add a fish to your display unless you are prepared to loose all fish in the tank. If it's the first fish in the tank, then OK. Doesn't matter where you buy the fish from; disease pathogens always are present and stress is the main contributor to fish illness.
How long should you QT? At lest a couple of weeks. In my case, it depends on which tank they are going to. If the new fish is headed for my fish only display, they may be in QT for up to 6 months. But then that DT contains a pair of Tinkers, pair of Interruptus, Black tang, Griffis angel, pair of Flame wrasse, Nahacky pair, psuedochromis pair and a yellow bellied regal.
Many folks think that QT is just to prevent disease, but it's much more. It's a chance to ensure that your new fish is feeding properly, as well as acclimating them to your choice of foods.
Do you add inverts or corals to your tank that contains fish? If so, you should QT those inverts to prevent the transfer of disease in the water. Yes... I QT snails and crabs going into my display tanks.
When it's time to add your new fish to the DT, use an acclimation box. They are readily available from your favorite LFS or can be a DIY if you are handy with acrylic. The acclimation box allows the tank inhabitants to become accustomed to the new addition, and helps reduce aggression when the bullies welcome the FNG (Fabulous New Guy). Properly design acclimation boxes can also be used as fish traps.
Coral QT is a separate discussion, but no less valid. Those same 10/20g tanks that are used for tank transfer when getting a new fish also work well for your new corals. Just add a light and dip away. Those damn pesky aiptasia somehow manage to slip through.......
You may hear from some that QT tanks just don't work because their fish always die, they simply haven't managed the QT process well. That or the fish/coral in question had issues to begin with that were too challenging for the aquarist to address or not recognized in time. Everyone looses the QT battle once in a while, just check your husbandry skills... it could have been the animal, the dosage or the lack of adequate nutrition. Often losses are a combination of factors.
Another nice link on QTs:
http://www.liveaquaria.com/PIC/article.cfm?aid=205