yes, it needs to be replaced. see they are brine shrimp, and they grow up. you cant keep them in there like you could phyto.
as far as being economical, yes, its very cheap. you can get the eggs and then you only hatch them once or twice a week as you desire
if you look at something like this
Premium Grade Brine Shrimp Eggs
you can get 8 to 16 Oz of premium eggs (only get premium eggs) for less than $30 and these will last you a ling time since you only use 1 tsp per hatch
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