When I first bought the plecos, they were 3 cm each, or roughly half the length of my little finger. That was eight years ago. Now they are about 25 cm, about as long as my forearm.
I let them go, because my tank has become too small for them. I really should have done this a year or two ago. However, I've been both lazy, and a little sad to say goodbye to these guys.
Eight years ago, I had bought the plecos, because of their tank-cleaning ability (plecos will suck and eat algae right off your aquarium glass walls). They are also easy to keep in a community tank, as they are completely docile and non-aggressive towards other fish. What I didn't expect was that my plecos would live so long and grow so big.
With hindsight, my purchase of the two plecos was a classic aquarist's mistake. The fish that you see on sale at aquarium shops are usually babies and juveniles. What you need to know is the maximum adult size of that particular species. If your tank is too small for a full-grown adult specimen to be swimming around freely in it (and if you don't want the hassle of upgrading to a larger tank), then you shouldn't buy that species of fish at all.
In the wild, plecos can reach a size that you'd never see in a home aquarium. See this monstrously large specimen:
Another classic victim of the size problem is the arowana. They are highly popular among Chinese businessmen, because they are believed to bring good luck and prosperity. On the other hand, most home aquariums are too small for an adult arowana to feel comfortable in it.
Also, arowanas are very powerful jumpers. In the wild, they have been reported to jump right out of the water to catch insects and small birds on overhanging branches.
In the home context, this means that one morning you might wake up to find that your arowana has jumped right out of its tank and is lying dead on your floor.
To raise arowana properly, you would need either a custom-made and very large fish tank, or a pond. All but the most serious aquarists should stay away from trying this species.
It's only when you see an arowana in a large open water area that you will truly appreciate what a beautiful fish it is. It moves with speed, power and grace - qualities which you won't really see if the arowana is stuck in a little tank.
And here's a fascinating video, showing two arowanas breeding. Stick around to the end, to see what the male does with the eggs! It isn't eating the eggs, it's keeping them in his mouth, to protect them. Really worth watching, if you're a fish enthusiast.