is it hard to have a salt water tank? No i would start at freshwater because when you get into saltwater, especially with rare tank fish you have to really keep an eye on your chemical balance and your temp. Also you can't stick a oscar in with something other than an oscar, no matter how big the fish is an oscar will eat it as long as theres more than one. except cichlads Its a hobby, and you will learn as you go but start with cheap fish, remember when it comes to filters you pay for what you get, same with heaters. You want a seahorse you better have some good equip. I think that jetmechG550 will agree with me on that. hope that helps a little