I was mentored by one of my father's friends who was probably a very typical "intermediate" hobbyist for that time period (1970's). He had a roomful of smallish tanks full of swordtails, angels, etc. and he warned me about a couple of things to beware of. I took some of it to heart right away, and some of it took a few years...or decades...before it sank in.
First...as stated by
O
Omrit
above...get the biggest tank you can reasonably afford and manage. Small tanks are a royal PITA in terms of stability. Big tanks suffer far fewer and far slower changes in parameters and are generally easier in every way to keep healthy.
Second...don't fall prey to the insane urge to own every species of fish in the world, and to squeeze yet another tank into every available space at your disposal. At some point, maintaining all those tanks and fish will quietly stop being a pleasant pastime and start being a chore, a major PITA. Don't let that happen. Keep it fun.
And finally...don't buy fish because they are "rare" or "desirable" or because nobody else has them. That's no reason for
you to want them. Decide what appeals to you and get fish that fit that mold. Who cares if everybody else has that species, or if they're common as dirt? Are you keeping fish for other people? Of course not; the point of this hobby is...pardon the repetition...to have fun. Choose the species that make you smile, keep them well, and you will have fun.
Incidentally, a corollary of that last comment is that as a beginner you should concentrate on fish that are suited to live in the water that is readily available to you, either from city mains or from a private well. Many hobbyists, especially advanced ones, keep fish whose water requirements are completely opposite to the water that comes out of their taps. Those folks either need to expend insane amounts of time, effort, space and money to treat their water to make it usable...or else they never have healthy fish. Either way...there's that whole "fun" thing going down the tubes...
Good luck; have fun!
