hey i was wondering how long after a fish has died can you add another one in the same tank after washing it out? thanks:)

So, Milo my fish died today, but I got a new betta and named him Pedro...so far thats the name...haha Anyways so I purchased an air pump tp fin 1 gallon tank, and Its all set up, and I added water conditioner and all. The water is even room temp. and ready! Can I add the fish....or do I wait a...