You didn't mention tank size, this will be part of what determines how many fish to have. The size difference you mentioned should be fine, as long as one doesn't fit in the others mouth.[email protected] said:I recently set up an all male peacock and hap tank. So far I have 9 fish all right at 3". I'm wanting to add 2 more, a Venustus and an OB peacock. The ones I'm wanting to order are 3.5" and 4". Is that too big to put in a tank with all 3" fish ?