So, yesterday I did my usual 50% water change on my 120. About 30 mins later I noticed a few of my smaller fish dead and a few of my bigger fish really struggling. I tested the water and had 3ppm ammonia, zero nitrite and around 10 ppm nitrate. I was totally confused, my tanks are all cycled and never have ammonia. I tested my tap water, sure enough, it read 3ppm ammonia! Any advice on how to proceed? I have two other tanks to change. BTW, I use Seachem Safe for water changes.