Depends on what type of acan it is, a lot of acan's were reclassified as micromussa which are a lot less aggressive. I would make sure to avoid Acan echinata's as they are very agressive to other acans. Some are fine being close with others, others have long sweeper tentacles. The same species type can usually touch.
Here is kind of a breakdown.
| Species | Typical Sweeper Reach | Aggression |
|---|
| Micromussa lordhowensis | 0.5–1.5" | Moderate |
| Micromussa amakusensis | 1–2" | Moderate |
| Acanthastrea echinata | 2–4"+ | High |
| Acanthastrea bowerbanki | 1–2" | Moderate |
| Homophyllia australis | 1–2" | Moderate |
| Micromussa pacifica | 0.5–1" | Low–Moderate |
SAFE TO KEEP TOUCHING OR VERY CLOSE (0–1")
• Micromussa lordhowensis ↔ Micromussa lordhowensis
• Micromussa lordhowensis ↔ Micromussa pacifica
• Micromussa pacifica ↔ Micromussa pacifica
Usually OK WITH ~1–2" SPACE
• Micromussa lordhowensis ↔ Micromussa amakusensis
• Micromussa lordhowensis ↔ Acanthastrea bowerbanki
• Micromussa pacifica ↔ Micromussa amakusensis
• Micromussa pacifica ↔ Acanthastrea bowerbanki
• Micromussa amakusensis ↔ Acanthastrea bowerbanki
• Homophyllia australis ↔ Micromussa lordhowensis
KEEP SEPARATED (3–4"+)
• Acanthastrea echinata ↔ anything
• Acanthastrea echinata ↔ Micromussa lordhowensis
• Acanthastrea echinata ↔ Micromussa amakusensis
• Acanthastrea echinata ↔ Acanthastrea bowerbanki
• Acanthastrea echinata ↔ Homophyllia australis