I cycled my 50 gallon and it was great, 0ammonia, 0 nitrites, 5 nitrates. but then I added 6 new platies and 6 guppies to it. 1 of them died the first night, so I didn't get much opportunity to see what was wrong, and I couldn't see anything. Few days later, and 2 start flashing. 2 of them have...