a huge factor is decorations. a 36" x 18" long tank potentially has more swimming room than someone elses 75 thats full of wood and decorations. nobody seems to talk about that too much on here. if you kept the clutter to a minimum id say youd be fine with large weekly waterchanges. IMO people are way overly agressive with stocking rules on every single fish forum. im not saying to put an oscar in a 40 breeder but id say a GT is fine in that tank no problem. well see how long it takes until someone decides to fight me over this.
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This is true...
As long as the volume is there (40g+) to dilute nitrates in between water changes, sufficient filtration, and a strict large water change schedule is kept, the fish should remain healthy. I honestly don't see that major of an issue with a fish that will reach 8-10" in 2-4 years in a 40 breeder. Who knows, by the time it reaches 6 the OP may upgrade as he sees fit.
A lot of people tend to pack their tanks with large driftwood/rocks, and 2-4 inches of substrate. All of that reduces the amount of actual gallons of water are in the tank, and reduce swimming room. It's good for large tanks (125g+) that can afford this luxury, or smaller ones with much smaller fish.
Another reason people suggest larger tanks is it allows less room for error. A lot of people want the biggest fish they can get for the tank. That's ok, but more times than not they rarely clean their filters or do water changes. A 75g tank with a rarely serviced filter and bi monthly water changes will still be bad, but not as bad as a 40b where the conditions will deteriorate much faster.
This was my GT, near 7", which I had for about a year in a 40g before I re-homed it. Barely 1" of substrate, and decor was minimal. He was always out and about. You can see the floating anubias which no matter how hard I thought I jammed it in there he'd dislodge it. A 75g would've been better, but it's not like he was suffering or miserable.
