If you can get some rock that is already cured you can put quite a bit in at a time. I'd probably go around 20 to 25lbs at a time, otherwise you need to find some way to cure it before putting in then you can put as much as you want. You can possibly even put more than the 20 to 25 but you will have a little die off regardless and with all those fish already there that would be my comfort zone.