Depends on how large the spawn is. I always limit the egg production to around 50 eggs (then I take out the female even if she is still "in the mood") so I can use a 5 gal tank until the babies get to be about 1/2"--then I move them to a 10 gal. tank (after that, they begin to need their own rooms; the girls stay in the 10 gal. and are soon distributed to other homes). I do have a 20 gal. but I've never had to use it--but I have 4 10 gals. that I use all the time (I can put 8 beanie boxes in a 10 gal. and only use 1 heater).