protein skimmers , salt water aquarium & brackish water

Question
-What kind of fish are brackish?  Are there any that are brakish and fresh water? I about throughly confused w/ this !   

If would set up a salt water aquarium it would be fish only.  Why can one only put 2 to 3 fish in a 20 gal. aquarium ---, are the slt water fish larger than fresh?  But, I have lookd at alot of salt water fish and some are not that big :-)  
Thanks again, Eileen------------------------
Followup To
Question -
I am thinkng about setting up a salt water aquarium. I now have a 20 gal.fresh water aquarium.  I have been trying to find if I can convert my fresh water over to a salt water using the equipment I'm presently using? I keep reading about protein sknimmers do I need one or not? Or is it used mainly for "good measure"?

I am also checking inot a brackish aquarium.  If would would can you help me to understand just what it really consists of. I do know that a brackish aquarium is salty but, not a "salt water aquarium"!  I also wonder what kind fish would "due" in such an aquarium.

Thanks so much!
Eileen

Answer -
Eileen,
I will always recommend the use of a protein skimmer, however on an aquarium 20 gallons or less you can get away without one as long as regular tank maintenance is performed (ie 25% water changes every three to four weeks, etc).

As far as using your other equipment... I guess that depends on what you have.  Hard for me to say yes or no when I don't know what you are currently using.  It will also depend on what you are hoping to accomplish with the tank.  Fish only?  Reef?  20 gallon aquariums are generally a bit small for saltwater as they are easily influenced and you can really only keep two to three smaller species in the aquarium.

Brackish set up should be treated the same as saltwater but without the skimmer because you use less salt.

Thank you for your time and best of luck with your tank!

Regards,
Scott Johnson
Aquarium Crazy Fish
www.aquariumcrazyfish.com

Answer
Eileen,
Brackish fish generally live where the freshwater rivers meet the salty oceans.  Some of these species can live in freshwater when they are juveniles and salt water when they are adults.  Generally they thrive in a semi-salted environment with species of a similiar need.  For a look at said species, I recommend you check out a local pet store or a book.

The fish that you are seeing in the pet store are generally juveniles.  The average marine fish grows to a size over 4 inches.  That being said, if you want your fish to grow up to be healthy you want to limit the number of fish you have.

Also, as the fish get older they tend to get a bit more territorial.  Assuming you pack them into an aquarium they will eventually become more and more aggressive until you start seeing dead bodies.

Mother Nature has her own way of dealing with overstocked aquariums.

Best of luck with your new hobby.

Regards,
Scott Johnson
Aquarium Crazy Fish
www.aquariumcrazyfish.com