Can a pet rat live in a fish tank? The short answer is no, while the long answer is yes, but only if you do very specific things to that will make is safe for your fuzzy friend.
To start off this laundry list of why you shouldn't keep your pet rats in a fish tank, you should know that rat urine gives off lots of ammonia which is deadly to your pet rat. The ammonia builds up very fast in an enclosed space like a fish tank without sufficient ventilation! And, it's hard to get proper ventilation when you're also doing your best to keep the little escape artist you have there inside the tank.
The second note comes down to ventilation as well, humidity build up can also be very detrimental to your pet rats health, as they actually have very sensitive respiratory systems. Did I mention it traps all the dust inside too? It's really just not good. So, unless you plan to clean out a stinky tank every day to every other day, I can't recommend it. Plus, the size of the aquarium you'd need to house two rats will very quickly get out of hand.
Take my 55 gallon tank that I got for my turtle awhile back, that was around $200. And that would be the size you should be looking at if you're going to have pet rats in a tank alone.
Now, with all that being said, there are a few solutions open to you if you have an old fish tank or aquarium sitting around that you'd like to use as TEMPORARY housing until you can setup a proper rat cage...
You'll need one that's a decent enough size. I'd say at the absolute smallest, a 20 gallon (and that's only big enough if you're going to use the massive add-on method pictured below) you could try to find a wire tank topper of sorts (like the bird cage used below). Essentially that's a cage that sits atop your fish tank, it will make is so that your ratties have some high spaces to climb up (which they LOVE), enough room to run around. This will also help ventilate the tank so humidity and dust don't build up.
The problem is going to be finding a topper big enough for your pet rats though, as these kinds of tank toppers really didn't catch on, and the ones that are available tend to be sized for hamsters, not rats. Look for ferret tubes or tunnels like these to connect it all together.
Can A Pet Rat live IN A Fish Tank?
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Which brings us right back to the original question. Can a pet rat live in a fish tank? Though technically possible, I certainly wouldn't recommend it for the plethora of reasons stated above. There are much better options, such as those we discuss here, especially if you are starting from scratch and aren't trying to work with stuff you already have.