Fresh or salt water?
I got into it way back in 1987 and have always had multiple tanks wherever I have lived.
Too many beautiful fish to just have one tank and since a lot aren`t compatible, we fishkeepers develop what we call "MTS"......"Multiple Tank Syndrome"
I have kept saltwater fish on many occasions....it isn`t hard like most people believe,it`s a bit different is all, as you have to mix your own water or buy it pre-mixed these days and measure it with a hydrometer to make sure the salinity level is correct.....I have also kept brackish water fish which are cool.
Presently,I have all fresh water tanks.......I really enjoy it......keeps me out of the bars/clubs !!

Lots of people have said to me that it`s cruel to keep them... what these novices don`t realize is that most fish are born and bred in tanks unless they are wild caught.