That's very true
They can be in the 10 gallon for now, but I highly recommend upgrading pretty soon. They are messy little fishies!!
The more the water, the more the ammonia is diluted, making it safer for them. I know tanks are pretty expensive, and you may not have money for a 40-80 gallon...
But did you know you could get deep tubs? clear ones even! They are pretty cheap, but you'd have to rig something up to keep the water clean for each tub, and the air circulating.
There is also a cheaper version, if you happened to have room indoors (though not recommended as not many have the room for such a contraption) is the blow up ponds.