how soon after adding fish can i do a water change?

I would start doing weekly water changes since the tanks already cycled with fish in it..
 
You can do your water changes once a week.

Since you have just cycled your tank, however, it is a good idea to really keep track of your ammonia, nitrites & nitrates since adding new fish can actually cause mini-cycles.

You will need to do water changes any time you ammonia or nitrites reach .25ppm - otherwise you might just kill your fish.

So I say, do water changes as often as you need to.
 
But only do about 25% today, because you don't want to lower the level of salt too quickly. You can put you snails in another container of water if your salt levels are too high for now.
 
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