It all depends on what you like and how you want to display it in your tank. A 40 gallon tank is really only big enough for two goldfish so if you go with that decision keep that in mind. The reason for this is the first goldfish needs 20-25 gallons for himself and any goldfish after that needs 15-20 gallons. This is because they are very messy fish and will fowl the water quickly. They also grow to a very big size and grow quickly.
An aggressive tank is fine and if that is what you want they can be quite awesome to look at. It seems these fish have a definite personality or attitude and they can be very entertaining to watch. If you choose African cichlids for example they are very colorful as well.
Most people prefer a community tank especially if they only have one tank they are dealing with. Most of the time this is the kind of tank that ends up being the most peaceful to watch. This is especially nice after a long day at work or if the day has been particularly hectic. I also think that the options for a peaceful community tank abound with this set up.
So you see, it is really up to you. Think about what you really want to get out of your fish tank. Research the kind of fish that you think you want or would like to have. Not every fish is appropriate for your size tank. Go to the LFS and look around at all of the different fish that are available to have in todays aquariums. Then come back to the net and to Aquaria Central to research what you think you would like to keep in your tank and we will be more then willing to answer all of your questions.
Good luck in your research and have fun with it. Oh and by the way, welcome to Aquaria Central. Have fun while you are here.
Marinemom
