20 gallon long aquarium?

I have a 20 gallon long aquarium. I had 3 dwarf hamsters and now simply 1 is still alive. I got them all at the same time and they are old-fashioned, about 1.5 years, so I'm sure the last one is going to see the bucket soon.

After my last one dies, I'd like to get some more small animals to replace my dwarves, but I'm not certain which type I should get.

What rodents would fit into a 20 gallon long aquarium comfortably? I would really like a jerboa but I've never seen anywhere to bring them, and I've looked.

So what's your opinion on what new rodent I should get?
What you do near hamsters on holiday?
Answers:    Mice, hamsters, gerbils.
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