From this previous topic and should apply here as well.
@Vernon Barry posted:I think you are talking about this accessory https://www.lionelsupport.com/Lionel-Aquarium
While I do not own one, I'm fairly familiar with the typical problems of many Lionel accessories and failing gears.
I would say a huge portion of the time, the failure is too tight of a press fit of the gear onto the motor or other shafting, coupled with splines or knurling of the shaft that cuts into the gear, and last- failure in the design to adhere to minimum number of tooth count for a given shaft bore size.
I tried looking up the gearbox, the motor, any picture I could find and really didn't get anywhere as to the gearbox that has failed in this aquarium.
If you can post a picture of the motor and or failed gear, we might be able to be more specific and find an exact gear.
That said, I use 2 things to fix many accessories: (Can be found on the "A" site)
#1 a tiny metric drill bit set with incremental sizes to bore out the gear ever so slightly to reduce the chances of splitting a new gear when pressed onto the shaft. "Unxuey 150Pcs Set Micro Drill Bit Set Power Tools Unxuey Mini Precision Hss Power Drill Small Precision Twist Drilling Kit with Plastic Box 0.4mm-3.2mm for PCB Crafts Jewelry Drilling Tool" Cost is $13 and money well spent.
#2 Then typically, I can find a gear with the proper teeth and modulus in an assortment set of plastic gears like this "58pcs Plastic Gears Module 0.5 for DC Motor DIY Model Toys" Cost is $7
Or this one is also a good set to have "AUKUYEE Plastic Gear Set, 75Pcs Single Double Reduction Gear Worm Gear for DIY Car Robot QY12" Cost is $13
And last another assortment: "GREEN FABWOOD 75 Type Plastic Crown Gear Single Double Reduction Gear Worm Gear (Original Version)" Cost $7
Example typical "can" style motor and the bushing one would oil.
