It really depends on the dimesions of the tank. I would say the minimum tank size should be a min of 48inches long by 18inches wide & 24inches high (75gallons). They are definitely shoaling fish though and should ideally be kept in groups of 5-6+ strong (though if you had a tank of the above stated measurements i wouldn't go for any shoal larger than this size). I know this sounds like a lot but this is the bare minimum size tank they should be in, in the long term- they grow rather large and are very active fish- when fully grown, seeing them shoal even in 100gallon tank makes it look a bit on the small side. So i would definitely say a 75-100gallon tank is the minimum tank size for bala/silver sharks

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