75g the dragon is about half a foot, but he is still really thin, and the pleco i have is pretty small
okay, so most of us think large predatory fish are super cool "because they usually are", so let's entertain housing one.
As you noted in the store you see many between 1/2 foot - 1.5 feet long. Obviously in the store they may be packed into smaller/medium sized tanks because the store probably isn't going to properly home them, as they were fish that haven't been sold and ended up living in the store, or they are fish that have been given up by owners because they could no longer house them.
Just remember when reading below this is in reference to adult size fish. Always consider the adult size when interested in housing a fish.
The "solo" or "paired" fish you might be able to house in a 75G tank I would say are going to be your SA cichlids. Oscars, JDs, Severums, and such will be able to manage in a 50G (1) , or 75-100G (2). They usually get no bigger than 12 inches, and I would say they can live in a tank 4 feet in length or so. They aren't "active" fish, now by that I do not mean they are docile, but usually active=swimming a lot, heavy movement... like bala sharks. for your tank I believe you said its 75Gs, I would do a Green Terror and any fish that it wont own. (green terrors are awesome fish, but they live up to their name) and if your looking at aggressive fish they might be a fun choice. a combo of an oscar + pleco would be ok too. You could also create a nice tank of African cichlids, as they are very bright and colorful, and do not grow as large. OH so you're probably wondering, why cant a 4 foot tank have 4 1foot oscars? oscars and large SA cichlids are usually very messy (large bioload), and aside from needing space to themselves, they pee and poo so much that being in a small cramped tank could kill/hamper their growth/lifespan. Imagine yourself a person confined to a 8x8 sq ft cubicle with another person, and a toilet that can only be flushed once a day... I would GAG after defacating!
lets push to the 100-200G limits. Here you have room for the large and active fish. Now we can talk bala sharks, columbian sharks, etc. 4-5 large cichlids also becomes suitable.
Let's talk about tanks ranging from 300-1000G (or ponds) seldom people will have tanks this large, but some are dedicated to their fish!
The large predatory fish people get into...
Redtail catfish, Irradescent sharks, Clown knifefish, Pacus, Arrowanas, Snakeheads (illegal), Shovelnose cats .... I could keep going
These guys are tank busters, and when they grow, they live up to the name! some can literally bust the walls of your tank due to being skittish or getting spooked!
The easiest types of aggressive fish to maintain and home I will say are prob going to be your SA cichlids.
When you go after large predatory fish you really have to consider:
1) cost of feeding (different diets as they grow)
2) cost of living (tank, filtration, maintainence) will not be cheap
3) space (do you have room for bigger and bigger tanks?)
4) age (tank busters live 10-20 years)
Now you could keep a "tank buster" in a small tank, for example a few people that recentlly joined have some in a 75G or a 50G, and they aren't 1-2 feet yet, but try to determine why you want one? do you want to grow a tank buster to live up to its name? do you want a cool fish to show off to your friends? If you are really fascinated by them you would consider the budget and care required to own one, and it's not pretty. If you just want one and don't looks to help it reach it's potential, then why bother?
