Rainbow sharks?

[absolutangel04]

There are other 'sharks' that you could put in a smaller tank. Like a red tailed black shark. I don't know their exact Requirements but I am pretty sure they don't get as large as a rainbow shark. (I did not read through your stock list so I can't comment on how it will do with your current set up)

The red tailed black shark is more aggressive than the rainbow shark and much more likely to wreak havoc on your tank. They are recommended for a 75g tank minimum most of the time, sometimes even that is not enough.

 

[OwduaNM]

The red tailed black shark is more aggressive than the rainbow shark and much more likely to wreak havoc on your tank. They are recommended for a 75g tank minimum most of the time, sometimes even that is not enough.

Maybe I was thinking of another kid of shark? I know I kept one 10 years ago in a 30 gallon tank but I can't remember exactly what it was called...but it's also possible that I was grossly misinformed back then.

 

[absolutangel04]

Maybe I was thinking of another kid of shark? I know I kept one 10 years ago in a 30 gallon tank but I can't remember exactly what it was called...but it's also possible that I was grossly misinformed back then.

It is also always possible that you found the one nice red tailed black shark out of every 100. lol.

For the OP, sometimes rainbow sharks do fine in 55g tanks, and honestly sometimes they don't. It depends on the individual fish. I would be aware before buying one that there is a chance it won't work out in the long term. If it something that your kid that might get too attached to, you may want to consider different fish.
Another consideration, what kind of frogs do you have?