Brackish water tanks are one of the hardest to keep.. There is no de-nitrifying stages, and it is also hard to keep live plants in Brackish water. Many of the Brackish water fish are very messy, and require heavy water changes, which is not easy with Brackish water.
Keep in mind you must be very tedious about your salinity levels, as nitrifying bacteria will only thrive at the same salinity. If you have a sudden shift in salinity, all of your beneficial bacteria will die off, causing an Ammonia spike.
Brackish Water is a term that means many ranges for most people.. Anywhere from 1.002-1.020 is Brackish, IMO. 1.020-1.025 is Saltwater range, as most fish can tolerate the lower salinity, but true seawater is 1.025.
You can buy a cheap hydrometer that goes all the way down to 1.001, I believe it is made by Kent.
Only use Marine Salt. Other types of salt will not create a 'brackish' environment.
Do yourself a favor and buy some 18g rubbermaid bins from Wal-Mart. The key to mixing Salt is to be consistant. The 18g rubbermaids will hold about 15g of water, so you can make a 'per 5g' measurement using this method. I often would just use 1 cup of marine salt, per 5 gallons... So 3 cups for a 15g rubbermaid. I would always pre-mix the brackish water, and have it prepared for water changes. Regardless of what salinity you choose, be consistant.
Brackish Water is sensative, so feed lightly. It is easy to overfeed some of the messier brackish fish, such as Green Spotted Puffers or Silvertip Sharks. When your Nitrates start to get high in Brackish, you can get some very bad algae blooms with the hardness in the water. Make sure you don't use intense lighting, unless you find some plants that will grow well in brackish water.
Make sure you over-filtrate.. It is always nice to give yourself a 'cushion' incase things go wrong. Always be very careful that nothing is decaying in a brackish tank, because it will cause major problems.
Use Mollys to cycle your brackish water.. They are brackish fish with a small bio-load.. Most of the larger brackish fish put off alot of ammonia, or are messy eaters.. They can also be sensative to Ammonia poisoning..
Another thing to remember, Ammonia is more toxic the higher the pH. With the hardness of the brackish water, your pH will be around 8.2-8.5, which makes Ammonia very lethal. That is why it is important to have a well cycled tank, and use inexpensive fish to FULLY cycle it. Always add fish SLOWLY in a brackish tank, so the nitrifying bacteria level can build up to the new amount of bio-load in the tank.
Hope that helps guys!
I have to go, but if I think of anything else, I'll be back!
Miles