About five weeks ago, I moved my single goldfish from a ten gallon tank to a 29 gallon tank. It’s an Eclipse system with a bio wheel. Testing told me that both ammonia and nitrogen were at zero, so I felt safe to go and buy a second fish.
Within 24 hours, the new fish was acting stressed, hiding behind the filter system and tilted at a 45 degree angle. Tests still read perfect, but I changed about about 20% of the water to be safe. Next day, fish 2 was no better, and my first fish seemed a little agitated in his movements. I changed about another 30% of the water over a period of a number of hours.
Today things are much better. My original fish is behaving normally and the new guy is getting there. I am going to just keep doing water changes every day. But this experience has me spooked. Both test kits (API brand) were purchased within the past six months from my local aquarium shop. Do these things sometimes just fail? It seems very clear to me that despite the readings, the water was not safe.
BTW, I removed the charcoal filter from the system a few days ago, as I remember reading here that these can sometimes seriously backfire.