since you have lots of room, when I was your age I had a fish room in my parents unfinished basement, I used kiddie pools for grow out tanks..raised angles. I used an oil filled heater, those were safer at least back in the 60's.
It sounds like it's a storage room under the stairs or something like that. You could definitely make it work.
One thing to consider is humidity. If there's no escape for the air it could get damp and cause mold issues. A dehumidifier may come in handy.
That all depends on the size of the room and the ventilation. I have to keep the door to my bedroom open in winter because there's absolutely no airflow and I can't open the windows (since it's usually below zero)! This is something you can address when it comes up but it's something to think about.
I think you can make a great fishroom out of it, no matter the size. Just make sure you can fit at least a lawnchair or beanbag chair in the center so you can enjoy your work!
There are also a ton of DIY projects you can do! I'm jealous!