Get a test kit, when the ammonia and nitrite readings are 0 then your tank is cycled. You should have some reading for nitrate. It takes about a month for the tank to be cycled, give and take. Be vigilant in the first few weeks, as you might do somethings inadvertently and throw off the cycle. Don't clean your filter and do your water change on the same day, don't add too many fish at once-these and other things can cause an ammonia spike.
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