Hi
I currently have 26 male bettas. All except the very very juveniles are kept in a minimum of 2 1/2 gallons, heated, sponge filters and live plants.

they make betta tanks with dividers here in the states by all glass aquariums. also the filters i have used were the simple sponge filters... little current.. massive bio filtration.

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This is not a direct attack on you guys in the states, but from what i've seen advertised there and in Australia being sold as betta tanks, well..............
I saw one that actually held 0.75 lts!!!!

these with solid, plastic dividers you make.. just drill some holes in them... this is a 2-1/2 gallon tank.
Round Dual-Action Foam Filter

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it would be better if it were bigger obv, but if the dividers were blacked out

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Kylie - The Godess Of Pop

the same can be done with any sized tank.. just cut some notches into the top frame and use opaque plactic panels cut to size.... works like a charm with minimal engineering. drill some small holes for circulation...

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Personally If I was keeping multiple bettas. I would run a system of tanks running of one sump based filter. That way you can seriously reduce the water current but keep the filtration efficiency up.

Same as habu 2.5-3 gallon minimum pref bigger. But remember these fellers are naturally from small amounts of water so you can't have a go at people who do keep them in small tanks. Obviously within reason but bettas will thrive in small tanks if its kept clean from toxins

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1.1 leos, 1.1 fat tails, 0.2 Beardies, 1 Whites, 2.0 Cresties, 4.1 Cats, Fish: 4.4 P. Dovii, 5.5.6 Peacock Bass (5 sp), 1.1 C 'Cobra', 1.1 festae, 1.1 green terror, 1.1 nics, 1.1 C. Marmorata, 1.1 C. Johani, 1.1 C. 'Strigata', 1.1 C. 'red tapojas'; 1.1 P. Friedrichsthalli, 1.1 Mots, 1.1 managuense, 1.1 H. Bocourti, 1.1 H. Pearsi, 1.1 H. Carpinte, 1.1 H. Cynoguttatum, 1.1 V. Argentea, 1.1 V. Zonatus, 1.1 N. Salvini, 2.2 Grammodes, 3.4 Hoplarchus Pssiticus, 2.2 Trimacs, 3.4 haitiensis, 1.1 tetracanthus, 3.2 Red tiger mots, 14 Uaru (2 sp) 1.1 Umbies, 2 Arapaima, 3 Red tail catfish, 1.1 P. Leopoldii, 1.1 P. Motoro, 3 Silver Arowana, Var. polypterus, var. Channa sp, 6 Pacu, M. wyckii, 3 siamese tigers, 2 jelly catfish, 1 Golden dorado

They are only in small amounts of water during the drought season

Only prob with a sump system running lots of tanks is if it goes wrong you could lose everything

Is that 2 1/2 gallons, imp or US? And is that 2 1/2 gallons in total?