You will be fine with a tsn. Or rtc. It's a decently large area and by the time they would out grow it they will ether be dead or you will upgrade regardless becAuse you will realize how great of pets they are. Not to mention if you don't buy them at the lfs someone with a 20gal will. Everyone who says u need a min of 5000gal or what ever is just not being realistic. Yes that would be nice but mine love there 1k pond. And I'll upgrade in the future but for now they are fine. Necro had 1 in a 1k pond that whas 3ft. So in closing I say do it. Just be ready for a large food bill.
PS. I ment dead as in something killed them ie aniamal, bird, desiese.... Not that the "small" area would kill them. Just to clarify