18" in a 55g is torture. As I said, if you got one @ 6" you'd have 6 months before it would need an upgrade at the most. There are many awesome fish that can go in your 90g, keep looking around unless you know you can get a huge tank soon.
For a general rule of thumb, a tank should be at least 1.5-2x deeper than a fish is long and 3-4x longer than a fish is long. This will keep them comfortable and happy.
So your 90g could house a single 12" fish at the maximum in my opinion. That opens many options.
For a general rule of thumb, a tank should be at least 1.5-2x deeper than a fish is long and 3-4x longer than a fish is long. This will keep them comfortable and happy.
So your 90g could house a single 12" fish at the maximum in my opinion. That opens many options.



