General

Hi, what would you suggest to test or review in order to discover why my cycle is crashing in all tanks that had a recent water change?

Have 3 display and 5 qt tanks running over 3-4 months without crashes. When I made small (<15%) water changes on 1 display and 3 qt tanks, the cycles eventually crashed. I didn't forget to use dechlorinator and only used the proper dosage. City water department says no chlorine flushes recently. They don't use chloramine.

I was worried about the fish, so doubled my filtestion with a seeded hob and sponge filter. Didn't seem to help. I was running out of ideas, so I tried some more and even a brand new, different brand of dechlorinator up to 5x the recommended dose, but still seems to cause crash. Water was still cloudy white. Also swapped out 80% of the water with Great Value Purified (supposely RO-trested) water. The water started clearing up next day. But, I had moved fish out the night before to qt since they started staying at the surface and I didn't think they would make it through the night.

For the 1 display tank, prior to the water change, I removed 3 of 4 endlers to sell and added 9 cherry shrimp after drip acclimating. These were added to preexisting 1 cherry, 3 amano, 10 endler fey and a dozen baby bladder snails.

The ammonia, nitrites and nitrates are 0. It seemed to me the dechlorinator wasn't working or the tap water has something in it killing the cycle. CO2 drop checker shows more O2 than co2. I don't co2 inject. Tank is moderately planted. I don't medicate in the display tank. Another odd thing was that PH was going down with each addition of dechlorinator. Is that supposed to happen?