Why You Should Keep Only One Betta
Written 2026-10-27
Male bettas are extremely territorial, so it’s safest to keep only one per tank. In our catalog, betta carries the highest aggression rating of any fish on the list — level 3. That said, two male bettas actually fighting each other is same-species aggression, and the Water Kiwoogi simulator doesn’t model that yet. So this guide can’t hand you a death count for "two bettas fight." Instead, we’ll show what the model does compute about betta’s aggression toward other animals, and why keeping one betta alone comes out clean.
Aggression level 3 — it targets small, slow tankmates
We put one betta with six cherry shrimp in a 30cm cube tank and ran it for 180 days. The betta stayed at 100% health the entire time, but the shrimp started dying on day 100, lost a second one by day 165, and ended day 180 with only 4 left, at 36% health. Run the exact same tank without the betta — six shrimp alone — and all six stay at 100% health for the full 180 days. So every one of those deaths comes from the betta’s aggression, rated 3. Betta’s predation cutoff is a 3cm body length, and cherry shrimp measure 2.5cm — right under the line.
| Scenario | Stocked | Alive at day 180 | First death | Last death |
|---|---|---|---|---|
| 1 betta + 6 cherry shrimp | 7 | 5 | Day 100 | Day 165 |
| 6 cherry shrimp alone (control) | 6 | 6 | None | None |
What the model can’t show yet — male-on-male fighting
Put two bettas together in that same 30cm cube and run it for 180 days, and in this model both stay at 100% health the whole time. That doesn’t mean two bettas get along — it means the simulator doesn’t yet account for aggression between individuals of the same species. In a real tank, two male bettas kept together will almost always fight, and the loser often ends up with torn fins or worse, especially in a small tank. We’re flagging this limitation up front: don’t read "both survived in the simulator" as "it’s fine to keep two."
One betta, kept alone, is the safe bet
Put a single betta alone in the same 30cm cube tank for 180 days and it stays at 100% health the whole way through. If you want a tankmate, pick something bigger than betta’s 3cm predation cutoff with low aggression of its own — that’s the only pairing the model itself backs up.
The button below is this guide’s example — a 30cm cube tank with one betta and no tankmate.
Run this tank yourselfThe numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.