They don't get huge - I think the larger tank size is mainly a recommendation if you're keeping a group where there might be some territorrial sqaubble. I keep 4 of them (2 M, 2 F) in my 46g. The dominant male will sometimes chase the other male, but they do no harm to each other, and usually just hang out in a group of 4. I know you could keep a M-F pair in a 5g and probably have no trouble, as that's the size many people use for breeding them, but I would say probably have 5g per killifish for adequate room for them.
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46g Bowfront, Filstar XP3, 149w, CO2 - Rainbows: 3 Red Irian, 3 Boesmani, 2 Turquoise; 5 Clown Loaches; 1 Dwarf Bristlenose Pleco; 1 Golden Wonder Killifish / heavily planted
25g Eclipse - 1 Common Pleco; dozen-or-so silver mollies / planted
5g flatback hex - 2 Goldfish (can't get rid of them)
1g triangle & 10g standard- Empty, used for breeding |