I have had all 3 types of tanks, plus indoor pond and river simulated ecosystems. If you have interest in saltwater I would say it would be a good time to start SLOWLY- set up on saltwater FOWLRs and reef tanks take 3 or 4 times the length of a freshwater set up, the fish are much less hardy and start up cost and maintenance are tougher than freshwater, but the fish are much more colorful, the variety of symbiotic relationships (coral reef tanks especially) and if your bioload and your CUC match up (coral reef again) your filtration maintenance and water changes are way less frequent. Brackish is a good introduction to saltwater, but is very labor intensive and livestock tend to be aggressive and finicky. You have had success with freshwater- after you treat the tank with a vinegar mix you could always expand on the tank you have already built, that way you have about half the water cycled already, and you would have some substrate and filter media already with bb- If you're going freshwater DO NOT use the saltwater filter, it's just asking for trouble. Is there any particular type of tank build that you're interested in already?