If it was set up since Nov it was cycled, but by switching out the substrate and and filter media all at once, you created a situation where the normal cycle was compromised, and then adding new fish compounded the problem. You may need to do daily or every other day water changes to dilute the ammonia and nitrite until it's fully cycled again. Don't vacuum every other day, simply remove enough enough water to dilute ammonia buildup.
+1. Same exact thing happened to me when I removed my sand and added a big Datnoid. Took about 2 weeks for my water to clear up with 20% water changes every other day
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