Depends on how much work you want to do- anything under 5 gallons will need 2 weekly water changes..
For filtered 1 50% water only and 1 50% substrate vacuuming per week.
For unfiltered 1 50% water only and 1 100% per week.
I go the cheap route for multiples until I can find more room to get them each big tanks-
The medium is 1.75 gals, the large is 2.75 gallons.. safe to heat with 25 watt adjustable heaters, can use a desk lamp with a 40 watt bulb to light them (not many desk lamps go higher in watt size, the 40w won't heat up the water, nor melt the plastic). There is enough room to plant it decently with a cave.