Did a 50% water change on my 29 gallon last night and as soon as I put the new water in i had 2 glass catfish spaz out and die within 1/2 hour. 3 more dead this morning. Thats 5 of 8 dead in 12 hours. The neon tetras and cherry barbs are happy as can be. I change water weekly so nothing changed from the routine. Tap water is normal. WHAT HAPPENED??