Me too, but I just had some FW eels a while back and am now just using the FW tank for housing guppies since I can't get shrimp for my seahorses yet. My LFS says you don't need to change the filter cartridges in them but that is probably BS since they also thought octos would eat aiptasia.
I think you would greatly benefit from visiting a store that sells salt water aquariums and equipment. You could have a look at things and have the sales person show you want goes with what size aquarium. I know you don't live near a big city, but maybe you could find a store and talk your parents into visiting it. You can see some of these things online, but its hard to get an idea of size and how they all work together.