Why Your Oscar Needs a Large TankOn OFL, and in general, it is recommended that you stock your tank as if all your fish are already adult size. This gives many people pause - a large 75-gallon tank with one, two-inch oscar in it can look very empty.
I know a few reasons for this, though, and I'm sure the rest of you know a whole bunch more.
1. They grow quickly
This is true of pretty much any cichlid, before you know it, that little two inch oscar is a hulking 10 inch fish, or larger (within a year). My little three-inch oscar is now a nine-inch oscar, and it's barely six months later. While it is theoretically possible to upgrade a tank as the fish grows, this is rarely practical, and definitely not cost effective.
2. Juveniles are more sensitive than adults to poor water quality
A big fish can take a lot of punishment. They can shake of maltreatment (albeit with other consequences, such as HITH) to a much greater degree than a little one can. Younger fish also have more to lose - if it doesn't grow properly (stunting due to poor water quality, for example) it will live a much-shortened lifespan and may never be able to handle diseases well. It's no secret that more water per fish = better water quality with the same amount of work. I dare say that because of this, a young fish needs more space than an adult fish of equal size (a three-inch oscar needs more water than a three-inch convict). (NOTE - I am in no way implying that you should maltreat your adult fish! There is an immense difference between surviving and thriving, and every fish deserves the best of care that we can manage)
3. Juveniles eat more, and poop more, than adult fish
As soon as fish are sexually mature, other preoccupations take precedence over eating and growing. Adult fish eat less (comparatively) and need food less frequently to be fine. Their metabolisms are slower. Now I'm not saying a two-inch fish needs more food than a twelve inch one, nor am I saying that a two-inch fish produces more waste than a foot long one. I'm simply saying that it is easy to underestimate the bioload of a growing fish.
4. Rehoming fish is never easy
Some people (many people) argue that they can just take back the extra fish when the tank gets too small. I have an incredibly helpful and friendly pet shop within half an hour of me. The manager and I are friends, and he'll happily take any fish that I cannot care for anymore. But I still hate doing it. I love my fish, they have names, personalities, habits - things I know because I spend a lot of time with them (and a whole pile of things that I probably imagine, much like people imagine that their cat can speak or that their dog can predict the future). Even when I raise fry for someone else, I worry - will they be well cared for? Now we haven't even considered cases where pet shops are not friendly, or where no one wants a medium sized oscar.
Rehoming is not really a simple, easy fix.
Imagine the reasoning if this were any other pet - I'll buy twelve kittens, and only keep one when they get too big in a few months.
Of course, there are many situations when this may not apply. I can think of two. Breeders growing out tiny fry may find a smaller tank is more suitable, to help the little ones reach the food in time, and to keep clean. Pet shops need to stock densely to have a large variety for sale. In both cases, neither place is planning to keep these fish for their entire lives happily and healthily. Daily water changes are also typically in effect (especially in the case of breeders).
Still, for me, the above makes a pretty convincing argument to stock as if your fish are already large.
Article by Nina W