jwh0818
Aquarium Advice Addict
Yes a 55g definitely is too small, just because of the 12" width. The 75g has an extra 6" in width which makes it a perfect home for a single full grown oscar. A 90g has 4" added in height which doesn't really make a difference because fish don't normally swim up and down. They swim left to right and the standard height of a 75g is still perfect because an oscar can easily look up and see his food. I'm not putting down a 90g for sure but I also wouldn't have it as a standard minimum for a single oscar, now if you notice your oscar getting 15, 16, 17" then yes, it's time for a bigger tank but the odds of that are really really slim. I for one am planning on upgrading my 75g to a 125g for just my oscar and a few featherfins. I don't like over stocking and I know what a messy pig an oscar is