From what I have witnessed, 2 Oscars in a 55 gallon would require
much more maintenance than a 20% change once a week. More like 30%+ every other day.
I do at least a 50% change once or twice a week in my 55 gallon with one O that has reached the 9-10" slow down size. We must also remember that the lower the nitrates the better, <20 doesn't mean keep it between 20 and 40, or keep it as close to 20 as you can ... over 20 is not healthy for long periods of time. <10 ppm is actually much better, exponentially.
But, yes, you can house more in smaller spaces with the best equipment set up and great attention paid to conditions, and providing the fish get along.
A 50 gallon breeder is wider than a 55 gallon for turn around room. Over-filtration. Almost daily large water changes, or a trickle system that constantly replaces water (like the 5-gallon oscar tank article - which used quite a large tank, btw).
With that being said, there is still a bottom limit for one, which I still feel is the 50 gallon breeder, just because it is a large fish and needs room to move, turn around, swim, to be healthy.
Here is a way to relate yourself to the fish, imagine being in a jail cell, like solitary confinement, but you
never leave the cell, and the toilet only flushes when the guards flush it. Now, add a second person to that cell.
That is essentially what you are doing when you put two Oscars in a 50 gallon. With utmost care and constantly keeping the cell clean, they may get along (and, if co-ed, they may get along very well and try to breed) ... stop flushing the toilet for them, and they may start to get cranky, mal-nourishment would cause problems, illnesses would be much easier to contract. We are but animals ourselves, don't forget that.
As far as born to breed ... I don't take that as seriously as some do. If you have owned or watched a lot of oscars before, you will know they have unique personalities, like humans. Some humans are anti-social, some are social butterflies, some are born to breed, some could care less about breeding.
Spreading seed to further the species isn't dominant in every animal. Studies have shown birds mate for life with a single partner. A lot of you have watched oscars pair up with a single other oscar.
Isn't love beautiful for those who find it?
The point made earlier, that in the wild, Oscars don't have constrictions. They are not confined to even thousands of gallons, but millions of gallons. A freshwater river. There are no nitrate creeping in, they are not restricted by an invisible barrier of glass.
I would love to house 6 oscars in a 300 gallon tank. I would just hate to do the water changes on it, since, the load would be just a bit larger than my single O in a 55 (6 x 55 = 330).
I also wouldn't mind keeping a single O in the same tank, just because I think one would love having all that space and territory to itself
So, the best rule is keep as many as you are willing to put effort into. Know the full grown size before you buy (not just being tempted by their baby cuteness).
here is a conversion 1 u.s. gallon = about 0.833 u.k. gallons. 1 u.s. gallon = about 3.78 liters.
I speak in u.s. gallons, since, I am on that side of the big pond.
Isn't it a shame (to the EU public) that the u.s. complains about paying less than $2.50 an imperial gallon/3.5 Liters? (1.72 Euros)
Quite spoiled we are! And, just a few years ago we were paying 1/3 of that
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