Two possible questions you could be asking here:
Can it literally fit into the tank? If that's what you're asking then yes, a smaller object does fit into a larger object generally.
Should you put it in there/will it be able to thrive? Nah that'd be a very silly question indeed, you must be asking the other one, please refer to my above answer if you need help with this.
Weird though - I thought children typically develop spatial understanding around 18 months, we've got some young posters on this forum so please keep it family friendly everyone!