Both sizes work. If you are choosing between them, buy the ten.
That is the short version, and the reasoning is not about swimming room. It is about how much a given mistake costs you.
The comparison
| 5 gallon | 10 gallon | |
|---|---|---|
| Adequate for one betta | Yes, this is the floor | Yes, comfortably |
| Ammonia after a missed change | Noticeably higher | Diluted, more forgiving |
| Temperature stability | Swings faster | Holds better overnight |
| Heater | 25 W, 50 W in a cold room | 50 W |
| Cycling | Straightforward | Straightforward |
| Weekly maintenance | About ten minutes | About ten minutes |
| Tank mates | Not sensibly | Possible, chosen carefully |
| Planting | Limited | Room for a proper layout |
| Purchase price | Lower | Modestly higher |
The row that decides it is the second one. Everything else is a smaller difference than people expect.
Why volume is forgiveness
Your fish produces ammonia continuously. In a cycled tank bacteria convert it, but the conversion has capacity limits and the concentration depends on volume.
The same amount of waste in five gallons is twice as concentrated as in ten. So a week where you skip a water change, or overfeed, or the filter clogs, produces half the problem in the larger tank.
Temperature works the same way. More water changes temperature more slowly, so an overnight drop in the room moves a ten gallon less than a five.
What ten gallons buys you beyond that
Room for a real planted layout, which gives the fish cover and gives you something better to look at than bare glass.
The option of tank mates. Snails almost always, shrimp often, a small shoal of pygmy corydoras sometimes. None of that is sensible in five gallons because there is no room to hold separate territories.
Somewhere for the fish to get away from the filter outflow, which matters for a weak swimmer with heavy fins.
When five is the right answer
When ten will not fit. A tank you cannot place is worse than a smaller one you can.
When weight is a concern. A filled ten gallon with substrate is roughly 100 lb, and not every piece of furniture will take that.
When the budget is genuinely tight. Five gallons run properly beats ten gallons bought at the cost of skipping the heater.
What not to do
Do not go below five because a listing says a betta will be fine. It will survive and it will not thrive.
Do not buy a tall ten gallon over a long one. Footprint and surface area matter more than height for a fish that breathes at the surface.
Do not buy the larger tank and then reuse the undersized heater. Match the heater to the volume.
Where to go next
The tank size pillar covers what volume actually changes, including the welfare guidance behind the five-gallon floor.